Unlocking Your 100k Potential: Determining How Long Your Long Run Should Be

The allure of the 100k ultramarathon is undeniable. It represents a monumental test of endurance, mental fortitude, and strategic planning. Sixty-two miles – a distance that pushes the boundaries of human capability. As you embark on the challenging yet rewarding journey of training for such an event, one question inevitably looms large, echoing in the minds of aspiring ultra runners: How long should my long run be for a 100k?

This isn’t just a casual query; it’s a cornerstone question that significantly shapes your training plan, influences your race-day readiness, and ultimately impacts your success in conquering the 100k distance. Get it right, and you build the physical resilience and mental confidence needed to cross the finish line. Get it wrong, and you risk injury, burnout, or falling short on race day.

The simple truth? There’s no single, magic number that applies to everyone. Unlike shorter distances where training formulas can be more prescriptive, the 100k long run demands a more nuanced, individualized approach. Factors like your running history, overall training volume, race goals, available time, and the specific nature of the 100k course itself all play crucial roles.

This comprehensive guide will delve deep into the complexities surrounding the 100k long run. We’ll explore the physiological and psychological reasons why it’s so critical, dissect the various factors influencing its ideal length, examine different training philosophies, and provide actionable strategies to help you determine how long your long run should be for your 100k. Prepare to go long, both in this article and in your training!

The Critical Role of the Long Run: Why It Determines Your 100k Success (And How Long It Should Be)

Before we dive into specific distances or durations, let’s firmly establish why the long run is the undisputed king of your 100k training schedule. Its importance cannot be overstated. It’s the session that most closely simulates the demands of race day and delivers a cascade of essential adaptations.

1. Building Aerobic Endurance and Cardiovascular Strength:

The primary goal is to enhance your body’s ability to sustain effort over extended periods. Long runs train your heart to pump blood more efficiently, increase capillary density in your muscles (improving oxygen delivery), and boost mitochondrial production (the powerhouses of your cells). This aerobic foundation is non-negotiable for surviving, let alone thriving, during a 100k. An appropriately challenging long run, addressing the question of “how long should my long run be for a 100k,” directly builds this core endurance.

2. Enhancing Fat Adaptation:

During prolonged, lower-intensity efforts, your body learns to become more efficient at utilizing fat as a primary fuel source, sparing precious glycogen stores. A 100k race is far too long to be fueled solely by carbohydrates. Effective long runs train this metabolic pathway, making you a more efficient endurance machine and reducing the risk of “bonking.” The duration of your long run directly impacts how effectively this adaptation occurs.

3. Strengthening Musculoskeletal Resilience:

Your muscles, tendons, ligaments, and bones need to withstand the repetitive impact of running for 10-20+ hours. Long runs gradually expose these tissues to stress, prompting them to adapt and become stronger, reducing injury risk. Figuring out “how long should my long run be for a 100k” involves finding the sweet spot that stimulates adaptation without causing breakdown.

4. Developing Mental Toughness and Problem-Solving Skills:

Ultramarathons are as much a mental battle as a physical one. Long runs provide invaluable practice in pushing through discomfort, managing fatigue, dealing with boredom, and overcoming the inevitable low points. You learn to troubleshoot issues (nutrition, hydration, gear) on the fly, building the psychological resilience needed to handle the challenges of a 100k. The sheer length of these runs forces mental engagement.

5. Practicing Nutrition and Hydration Strategies:

A 100k cannot be completed without a solid fueling and hydration plan. Your long runs are the perfect laboratory to experiment with different foods, gels, electrolytes, and fluids. You learn what your stomach can handle, how often you need to refuel, and how much fluid you require under varying conditions. Dialing this in during training prevents disastrous gut issues on race day. The answer to “how long should my long run be for a 100k” must factor in sufficient time to practice these critical elements.

6. Testing Gear and Logistics:

Shoes that feel great for an hour might cause blisters after four. A hydration pack that seems comfortable initially might chafe relentlessly after six hours. Long runs are essential for testing every piece of gear – shoes, socks, shorts, shirts, packs, headlamps, poles – under race-like duration and conditions. This minimizes surprises and discomfort during the actual 100k.

7. Pacing Practice:

Learning to control your effort level early in a 100k is paramount. Going out too fast is a classic ultra mistake. Long runs provide the opportunity to practice sustainable pacing, understand how different effort levels feel over time, and learn to run by feel rather than solely relying on a watch.

Understanding these benefits underscores why the long run is fundamental. It’s not just about logging miles; it’s about triggering specific, crucial adaptations essential for 100k success. Now, let’s tackle the core question head-on.

**Defining Your Peak 100k Long Run: How Long Should It Actually Be?**

So, we arrive at the million-dollar (or perhaps, sixty-two-mile) question: what’s the target distance or duration for your longest training run(s) leading up to a 100k?

As mentioned, there’s no universal answer. However, we can explore common ranges, philosophies, and the rationale behind them.

Common Ranges and Philosophies:

The Time-on-Feet Approach: Many experienced coaches and ultrarunners advocate focusing on duration rather than distance, especially if training for a hilly or technical 100k where pace can vary dramatically. The goal is to accustom your body to being active for extended periods. For a 100k, peak long runs using this method often fall in the 6 to 9-hour range. This approach automatically adjusts for terrain difficulty – a tough 6-hour mountain run might cover significantly less distance than a flat 6-hour road run, but the physiological stress and time spent adapting are comparable.
The Percentage of Race Distance Approach: Some runners think in terms of covering a significant fraction of the total race distance. For a 100k (62 miles), common recommendations often suggest peak long runs in the range of 50% to 70% of the race distance. This translates to roughly 31 to 43 miles (50km to 70km). Running much beyond this percentage in a single session often yields diminishing returns regarding fitness gains while exponentially increasing injury risk and recovery time needed.
Mileage-Based Caps: Often tied to the percentage approach, some plans simply cap the longest single run at a specific mileage, regardless of the exact percentage. Common caps you might see are 30-35 miles (around 50-56km) or sometimes up to 40 miles (64km) for very experienced runners training at high volumes. The idea is that runs longer than this provide minimal additional physiological benefit compared to the recovery cost.
The Back-to-Back Long Run Strategy: This is a very popular and effective method in ultra training. Instead of doing one enormous long run, you split the stress over two consecutive days. For example, running 20-25 miles (32-40km) on Saturday followed by 15-20 miles (24-32km) on Sunday. This approach simulates running on fatigued legs (a key aspect of ultras), accumulates significant volume and time on feet, but often allows for quicker recovery than a single, much longer effort. Total weekend volume might reach 35-45 miles (56-72km) or even more, achieved over two days. This method significantly influences the answer to “how long should my single long run be for a 100k,” as the peak single run might be shorter if back-to-backs are employed effectively.

Why Not Run 100k (or close to it) in Training?

You might wonder why training plans don’t typically include runs close to the full 100k distance. The reasons are primarily centered around recovery and injury risk:

Excessive Recovery Time: A run of 80km, 90km, or 100km would require weeks, potentially even a month or more, to fully recover from. This recovery period would significantly disrupt your training schedule, negating any potential fitness benefits by forcing extended downtime right when you should be finalizing preparations.
High Injury Risk: The sheer stress of running such extreme distances in training drastically increases the likelihood of developing overuse injuries (stress fractures, tendonitis, etc.). A serious injury close to race day could derail your entire attempt.
Diminishing Returns: The physiological stimulus needed for 100k adaptation can generally be achieved with runs in the 50-70km (or 6-9 hour) range, or via well-structured back-to-back runs. Pushing significantly beyond this in a single session offers little extra fitness benefit while massively increasing the associated risks and recovery demands.

The goal of the peak long run isn’t to prove you can run 100k before the race; it’s to stimulate the necessary adaptations and build confidence, allowing you to arrive at the start line healthy, recovered, and prepared to utilize your training on race day.

Key Factors Defining How Long Your 100k Long Run Should Be

Now, let’s personalize this. The ideal length of your peak long run depends heavily on you and your race. Consider these crucial factors:

1. Your Running Experience and History:

Beginner Ultrarunner: If this is your first 100k (or even your first ultra beyond 50k), caution is key. Your body is still adapting to ultra-distances. Focusing on consistency and gradually building your long run is more important than hitting an arbitrary massive number. A peak single long run towards the lower end of the ranges (e.g., 50km / 31 miles, or 6-7 hours), potentially supplemented by back-to-backs, is often sufficient. Priority: Finish healthy.
Experienced Ultrarunner: If you have multiple ultras, including 100ks or longer, under your belt, your body is more resilient. You might handle, and potentially benefit from, slightly longer peak runs (e.g., 60-70km / 37-43 miles, or 8-9 hours), especially if targeting a specific performance goal. You also have a better understanding of your personal recovery needs. Priority: Optimize performance based on past experience.

2. Your Overall Weekly Training Volume:

How long your long run should be for a 100k is intrinsically linked to your total mileage. The long run shouldn’t represent an excessively large percentage of your weekly total, as this increases injury risk.

Lower Volume Runners (e.g., 40-50 miles/week or 65-80 km/week): A peak long run of 30 miles (50km) already represents a huge chunk of the weekly volume. Pushing much further might be counterproductive. Back-to-backs might be a safer way to get more time on feet.
Higher Volume Runners (e.g., 70+ miles/week or 110+ km/week): Can more safely incorporate longer peak runs (e.g., 35-40 miles / 56-64km) as it represents a smaller relative stress compared to their total training load. Their bodies are generally better conditioned to handle the mileage.

General Guideline: Aim for your peak long run(s) to be no more than roughly 30-40% of your peak weekly mileage, although this is a loose guideline and can be exceeded, especially if using a time-on-feet approach or incorporating significant walk breaks.

3. Time Available for Training and Recovery:

Long runs demand significant time, not just for the run itself but also for preparation and recovery. A 7-hour run requires most of a day. Be realistic about your schedule. If time is tight, well-structured back-to-back runs might be more feasible and effective than one extremely long single session that leaves you wrecked for days. Adequate sleep and recovery practices are paramount – factor these in when deciding how long your long run can realistically be.

4. Your 100k Race Goals:

Goal: Completion: If your primary aim is simply to finish the 100k, focusing on consistency, staying injury-free, and achieving solid time-on-feet (perhaps via back-to-backs) is often sufficient. You might not need to push the absolute upper limits of long run distance. Mastering nutrition and pacing during moderately long runs (e.g., 50k) is crucial.
Goal: Performance/Competitive Time: If you’re aiming for a specific time or placing, you might need to incorporate longer or more intense peak long runs, potentially closer to the 60-70km (or 8-9 hour) mark, and likely including segments at goal race pace effort. This requires a higher overall training volume and careful attention to recovery.

5. Race Terrain and Profile (Specificity is Key!):

This is hugely important. How long should your long run be for a flat 100k versus a mountainous 100k?

Flat/Road 100k: Pace will be more consistent. Long runs might focus more on distance covered and practicing steady pacing. Peak runs might reach 60-70km. The repetitive stress on specific muscle groups is high.
Hilly/Mountainous/Technical Trail 100k: Pace varies dramatically. Time on feet becomes a more relevant metric than pure distance. Training must include significant elevation gain/loss and practice on similar technical terrain. A 7-hour mountain run might only cover 45-50km but could be far more demanding than a 60km flat run. Incorporating power hiking on uphills is crucial. Gear testing (poles, rugged shoes) is vital.
Matching Training to Course: Your longest runs should, as much as possible, mimic the predominant terrain and elevation profile of your target race during the specific phase of training.

6. Your Injury History and Recovery Capacity:

Listen to your body! If you are prone to certain injuries, be more conservative with your peak long run distance. Prioritize consistency and staying healthy over hitting a specific number. Pay close attention to how quickly you recover from long efforts. If a 30-mile run takes you a week to feel normal again, pushing for 40 miles might be unwise. Chronic fatigue is a sign you might be doing too much. Optimize sleep, nutrition, and active recovery (stretching, foam rolling).

By carefully considering these factors, you can move from generic advice to a personalized strategy for determining how long your long run should be for your specific 100k attempt.

Structuring Your Long Runs Throughout the 100k Training Cycle

Your long run distance isn’t static; it evolves throughout your training plan, typically spanning 16-24 weeks (or longer for beginners). Understanding this progression is key.

1. Early Base Building Phase (First ~4-8 weeks):

Focus: Consistency, building aerobic base, gradually increasing mileage.
Long Run Goal: Slowly extend the duration/distance of your longest run of the week. Don’t rush. Increase by no more than 10-15% per week, and consider incorporating “down” weeks (with a shorter long run) every 3-4 weeks to allow for adaptation.
Example: If you start comfortably running 10 miles, you might progress 10 -> 11 -> 12 -> 10 (down week) -> 13 -> 14.5 -> 16 -> 13 (down week).
How Long Should It Be? Relatively short compared to peak runs, maybe reaching 15-20 miles (24-32km) or 2.5-3.5 hours by the end of this phase.

2. Mid-Training Phase (Build Phase – Next ~6-10 weeks):

Focus: Significant increases in long run distance/duration, introducing race-specific elements (terrain, hills), start practicing nutrition/hydration seriously. Introduction of back-to-back long runs if using that strategy.
Long Run Goal: This is where the bulk of the long run progression happens. You’ll push towards your target peak long run range. Increases become more substantial but still require careful monitoring.
Example: Progressing from 20 miles, you might see weeks like: 20 -> 22 -> 25 -> 20 (down) -> 27 -> 30 -> 32 -> 25 (down) -> 35 (potentially a peak run). Or, incorporating back-to-backs: Sat 18/Sun 12 -> Sat 20/Sun 14 -> Sat 22/Sun 16 -> Sat 25/Sun 18 (peak weekend).
How Long Should It Be? Steadily climbing towards your personalized peak range, likely hitting runs in the 25-35 mile (40-56km) or 4-6 hour range regularly.

3. Peak Long Run(s) Phase (Typically 3-6 weeks before race day):

Focus: Executing your longest, most demanding training run(s). This is the final big stimulus before the taper. Confidence building is key.
Long Run Goal: Hit your planned maximum long run distance or duration, based on all the factors discussed. This might be a single longest run or the longest back-to-back weekend.
How Many Peak Runs? Usually 1 or 2 peak efforts are sufficient. Doing too many very long runs close together increases burnout risk. These runs should be spaced 1-2 weeks apart.
How Long Should It Be? This is where you hit your 50-70km / 31-43 mile / 6-9 hour single run, OR your longest back-to-back combination (e.g., 30 miles Sat / 20 miles Sun). This run should ideally be done 3-4 weeks before the 100k. Some plans might have a slightly shorter “last long run” 2-3 weeks out.

4. Tapering Phase (Final 2-3 weeks before race day):

Focus: Recovery, absorbing training, sharpening, staying healthy. Reduce overall volume significantly.
Long Run Goal: Drastically reduce the length of the long run while maintaining some intensity. This allows your body to fully recover and rebuild before the race.
Example: If your peak was 35 miles 3 weeks out, your long run 2 weeks out might be 15-20 miles, and the weekend before the race might just be 8-10 miles or even less.
How Long Should It Be? Significantly shorter. Resist the urge to cram in more long runs during the taper – the work is done! Focus on feeling fresh.

This structured progression ensures you build the necessary endurance gradually, minimizing injury risk while maximizing adaptation, leading you to confidently answer “how long should my long run be for a 100k” at each stage of your training.

**Beyond Distance: The Quality of Your 100k Long Run Matters Immensely**

While the question “how long should my long run be for a 100k” focuses on quantity, the quality of those long runs is equally, if not more, important. A poorly executed long run, regardless of its length, offers limited benefit. Here’s what defines a high-quality long run for 100k prep:

1. Appropriate Pacing:

Majority Easy/Conversational: Most of your long run volume should be at a comfortable, conversational pace (Zone 1-2 heart rate). This is crucial for building the aerobic base and fat adaptation without excessive stress.
Incorporating Goal Effort (Strategically): Depending on your goals and experience, you might include segments at your anticipated 100k race effort/pace within your long runs, especially in the later stages of training. This helps your body learn how that effort feels over time. Avoid doing the entire long run at race pace – that’s too stressful.
Practicing Downhills: If your race has significant descents, practice running them efficiently during long runs to condition your quads.
Power Hiking Uplills: For hilly courses, integrate power hiking on steep climbs – this is a critical skill for conserving energy in an ultra.

2. Dialing in Nutrition and Hydration:

Start Fueling Early and Often: Don’t wait until you’re hungry or thirsty. Begin taking in calories (gels, chews, real food) and fluids/electrolytes within the first 30-60 minutes and continue consistently throughout the run (e.g., 200-300 calories per hour, adjusted individually).
Experiment: Use long runs to find out what foods and drinks work best for you. What settles well? What provides sustained energy? What causes GI distress? Keep detailed notes.
Simulate Race Day: Try to use the types of fuel and hydration that will be available at aid stations if you plan to rely on them, or practice carrying and accessing everything you’ll need if self-supporting.
Practice Drinking: Learn to drink effectively while moving, whether from bottles or a bladder.

3. Comprehensive Gear Testing:

Head-to-Toe Check: Run in the exact shoes, socks, shorts, shirt, pack, and hat you plan to use on race day. Identify any potential chafing, blister hotspots, or discomfort issues before they ruin your race.
Pack Practice: Load your pack with the gear you’ll carry (water, food, jacket, headlamp, mandatory gear) and run with it to get used to the weight and fit. Practice accessing items on the move.
Headlamp Use: If your 100k involves running in the dark (highly likely), do some long run segments at night to test your headlamp’s brightness, beam pattern, battery life, and comfort.
Trekking Poles: If you plan to use poles, practice with them extensively during long runs on relevant terrain.

4. Terrain Simulation:

Match the Course: As much as feasible, seek out terrain similar to your race course for your key long runs. If it’s a technical, rocky trail race, spending all your time on flat pavement is poor preparation. Find trails. If it’s hilly, run hills. If it’s flat and runnable, practice sustained running on similar surfaces.
Vertical Gain/Loss: Pay attention to the elevation profile. Try to replicate similar amounts of climbing and descending per mile/km in your relevant long runs.

5. Mental Preparation and Strategy:

Embrace Discomfort: Use long runs to practice your mental coping strategies. Acknowledge low points, but work through them. Break the run down into smaller, manageable segments.
Positive Self-Talk: Develop mantras or positive affirmations to use when things get tough.
Problem Solving: View unexpected issues (weather change, gear malfunction, feeling low) as opportunities to practice problem-solving under fatigue, just like you’ll need to do during the 100k.

A 30-mile long run where you nail your pacing, nutrition, gear, and mental game is far more valuable preparation for a 100k than a sloppy, poorly fueled 40-mile death march that leaves you injured or demoralized. Focus on quality execution within the context of determining how long your long run should be for your 100k.

The Strategic Advantage: The Role of Back-to-Back Long Runs for 100k Training

We touched on back-to-back long runs earlier, but they deserve a dedicated section because they are a cornerstone of many successful 100k training plans and directly influence the discussion around “how long should my single long run be for a 100k.”

What are Back-to-Back Long Runs?

Simply put, running two significant long runs on consecutive days, typically Saturday and Sunday.

Why Use Them for 100k Training?

Simulating Race Fatigue: The primary benefit is teaching your body and mind to perform on already tired legs. Sunday’s run starts with the residual fatigue from Saturday’s effort, mimicking the feeling you’ll experience deep into a 100k race.
Accumulating Volume Safely: They allow you to log significant mileage or time-on-feet over a weekend without the extreme stress and prolonged recovery associated with a single, massive run (e.g., 60-70km+). A Sat 30km / Sun 25km might be more manageable and repeatable than a single 55km run.
Enhanced Fat Adaptation: Two consecutive long, lower-intensity efforts can further stimulate your body’s ability to utilize fat for fuel.
Mental Toughness Boost: Starting a long run when you’re already tired builds incredible mental resilience. Learning to push through this discomfort is invaluable ultra training.
Time Efficiency: For those with busy schedules, fitting two moderately long runs might be easier than carving out the 6-9+ hours needed for a single peak run.

Structuring Back-to-Back Long Runs:

Typical Distances/Durations: For 100k training, common back-to-back combinations during the peak phase might look like:
- Saturday: 20-30 miles (32-50km) or 3-5 hours
- Sunday: 15-25 miles (24-40km) or 2.5-4 hours
Intensity: Generally, both runs should be at an easy, conversational effort, especially the second day. The focus is on duration and coping with fatigue, not speed.
Progression: Introduce back-to-backs gradually. Start with shorter distances (e.g., Sat 12 miles / Sun 8 miles) and build up over several weeks.
Frequency: You might incorporate back-to-back weekends every 2-3 weeks during the main build phase, culminating in your longest back-to-back effort 3-5 weeks before the race.
Recovery is Crucial: Pay extra attention to nutrition, hydration, and sleep between the two runs and in the days following.

How Back-to-Backs Influence Your Single Longest Run:

If you effectively incorporate challenging back-to-back long runs into your plan, you might find that your single longest run doesn’t need to be at the absolute maximum end of the suggested ranges (e.g., 40+ miles / 65km+). A peak single run of 50-60km (31-37 miles), combined with peak back-to-back weekends reaching a total of 60-80km (37-50 miles) over two days, can provide a very robust stimulus for 100k readiness with potentially less injury risk and faster recovery than a single 70km+ effort.

Ultimately, deciding whether to prioritize a single peak long run or utilize back-to-backs (or a combination) comes back to the individual factors: experience, time, goals, and how your body responds.

Common Mistakes When Planning Your 100k Long Run (And How to Avoid Them)

Even with the best intentions, runners can stumble when planning and executing their crucial long runs. Being aware of common pitfalls helps you navigate your training more effectively.

Doing Too Much, Too Soon: The most common mistake. Ramping up long run distance or duration too quickly without allowing the body adequate time to adapt.
- Avoidance: Follow the ~10-15% weekly increase guideline (for the long run), incorporate down weeks, and listen to your body’s feedback (niggles, fatigue). Be patient; building ultra endurance takes time.
Making the Long Run Too Large a Percentage of Weekly Volume: A 30-mile long run in a 40-mile week is asking for trouble.
- Avoidance: Ensure your overall weekly mileage builds progressively alongside your long run. Aim for the long run to be roughly 30-40% (or less) of your peak weekly volume. If you want longer long runs, you need higher overall weekly mileage to support them.
Ignoring Recovery: Running a long effort and then not prioritizing sleep, nutrition, and rest afterwards.
- Avoidance: Treat recovery as part of the training. Focus on refueling immediately post-run, hydrating well, getting adequate sleep (especially after long runs), and considering active recovery like gentle stretching or walking.
Not Practicing Nutrition/Hydration: Going out for a 5-hour run with only one gel and a small bottle of water.
- Avoidance: Use EVERY long run (over ~90 minutes) as an opportunity to practice and refine your race day fueling and hydration strategy. Start early, fuel often, and experiment to find what works.
Failing to Simulate Race Conditions: Doing all your long runs on flat pavement when the race is a technical mountain trail, or never running with your race pack.
- Avoidance: Prioritize specificity, especially in the later stages of training. Run on similar terrain, practice with hills (up and down), test all your gear, and run segments in the dark if necessary.
Running Long Runs Too Fast: Trying to smash every long run or comparing your pace to others.
- Avoidance: Keep the vast majority of long run miles at an easy, conversational effort (Zone 1-2). Save faster running for specific, planned workouts. Remember, the primary goal is time on feet and endurance building, not speed.
Obsessing Over a Specific Number: Feeling like a failure if you “only” ran 30 miles for your peak run when someone else ran 40.
- Avoidance: Remember individualization! Focus on your plan, your progress, and your factors. Consistency and staying healthy are more important than hitting an arbitrary distance. Trust your training.
Skipping the Taper: Doing a very long run too close to race day.
- Avoidance: Respect the taper! Your longest effort should typically be 3-4 weeks before the 100k. Drastically reduce long run distance in the final 2-3 weeks to allow for full recovery and peak performance.

Avoiding these common errors significantly increases your chances of arriving at the start line healthy, confident, and truly prepared.

Listening to Your Body: The Ultimate Guide in Your 100k Journey

Amidst all the plans, percentages, and philosophies about “how long should my long run be for a 100k,” the most crucial piece of advice is deceptively simple: listen to your body.

Your body provides constant feedback. Niggles, persistent fatigue, poor sleep, lack of motivation – these are signals that you might be pushing too hard or need more recovery. Ignoring these signals is a fast track to injury or burnout.

Be Flexible: Don’t be rigidly attached to the plan. If you feel excessively fatigued or a minor ache is worsening, it’s okay to shorten a long run, swap it for cross-training, or take an extra rest day. Missing or modifying one session is far better than developing an injury that sidelines you for weeks.
Distinguish Good Pain from Bad Pain: Muscle soreness and fatigue after a long run are normal (good pain). Sharp, localized, or worsening pain, or pain that alters your gait, is not (bad pain). Learn to recognize the difference.
Monitor Subjective Feelings: Pay attention to your energy levels, mood, appetite, and sleep quality. These are often the first indicators of overtraining.
Embrace Rest Days: Rest is when adaptation happens. Don’t view rest days as unproductive; they are essential components of successful training.

Your intuition and subjective feeling, combined with objective data (like heart rate, if you use it), are powerful tools. Trust them. The perfect plan on paper is useless if it breaks you. The best plan is the one that gets you to the start line healthy and prepared, and that always involves tuning in to your body’s signals.

Illustrative Sample Long Run Progression (Highly Generalized)

Disclaimer: This is a highly generalized example for an intermediate runner targeting completion, assuming a peak run 3-4 weeks out and incorporating some back-to-backs. It MUST be adapted based on individual factors.

Weeks 1-4 (Base): 10 -> 12 -> 14 -> 11 (miles)
Weeks 5-8 (Build 1): 15 -> 17 -> 19 -> 15 (miles)
Weeks 9-12 (Build 2): 21 -> 23 -> Sat 15/Sun 10 -> 18 (miles)
Weeks 13-16 (Build 3 / Peak): 26 -> Sat 18/Sun 12 -> 30 -> Sat 22/Sun 15 (miles)
Weeks 17-20 (Peak & Early Taper): 32-35 miles (Peak Single Run) -> Sat 25/Sun 18 (Peak B2B) -> 20 miles -> 15 miles
Weeks 21-22 (Taper): 10 miles -> 6-8 miles (easy)
Week 23 (Race Week): Very short shakeout runs. RACE DAY!

Again, this is purely illustrative. Your progression might be faster or slower, use different distances, focus more on time, or rely solely on single long runs or back-to-backs. Work with a coach or adapt carefully based on your experience and response to training.

Frequently Asked Questions (FAQ) About 100k Long Runs

Based on common questions runners ask when preparing for a 100k:

1. What is the absolute longest run I should do before a 100k?

There’s no single “absolute” number, but common practice suggests peak single long runs often fall between 31 miles (50km) and 43 miles (70km), or 6 to 9 hours of time on feet. Experienced, high-volume runners might occasionally touch the upper end or slightly beyond, while many runners find success capping around 30-35 miles (50-56km), especially if using back-to-back long runs effectively. Running much further significantly increases recovery time and injury risk with diminishing returns.

2. How many miles should I run a week for a 100k?

This varies greatly based on experience and goals. For completion, many runners build up to peak weeks of 40-60 miles (65-100km). For more performance-oriented goals, peak weekly mileage might be significantly higher, often 70-100+ miles (110-160+ km). Consistency and the quality of key workouts (like the long run) are often more important than hitting an arbitrary high number, especially if it leads to injury. Your long run should generally not exceed 30-40% of your total weekly volume.

3. Is 50k (31 miles) the longest run I need for a 100k?

For many runners, especially first-timers or those primarily aiming for completion, a peak long run of 50km (31 miles), potentially combined with strong back-to-back long run weekends, can absolutely be sufficient preparation for a 100k. It provides a significant physiological stimulus and boosts confidence without the extreme recovery demands of much longer runs. You don’t have to run longer than 50k in a single session to successfully finish a 100k, provided the rest of your training (volume, consistency, specificity) is solid.

4. Should I do back-to-back long runs for a 100k?

Back-to-back long runs are a very popular and effective strategy for 100k training. They help simulate race-day fatigue, allow for accumulation of significant time on feet with potentially less risk than a single massive run, and build mental toughness. While not strictly mandatory (some succeed with single peak runs), they are highly recommended, especially if you can’t feasibly or safely execute single runs in the 6+ hour / 35+ mile range, or if you want an extra edge in preparing for the later stages of the race.

5. What pace should my long runs be for a 100k?

The vast majority of your long run miles should be at an easy, conversational pace (often 1-3 minutes per mile slower than marathon pace, or Zone 1-2 heart rate). This builds aerobic endurance and fat adaptation efficiently. You can strategically incorporate some segments at your projected 100k race effort (not necessarily exact pace, especially on trails) in later long runs to practice, but avoid running the entire duration too fast. Focus on sustainable effort over speed. Remember power hiking hills!

6. How far out from the 100k race should my longest run be?

Your peak long run(s) should typically be completed 3 to 4 weeks before your 100k race. This allows sufficient time for your body to recover from the effort, adapt, and absorb the training stimulus during the taper period. Doing your longest run any closer (e.g., 1-2 weeks out) risks starting the race fatigued and compromises the vital tapering process.

Conclusion: Finding Your Ideal 100k Long Run Length

Determining how long your long run should be for a 100k is a critical piece of the ultramarathon puzzle, but it’s one with a personalized answer. There is no magic number, only principles and guidelines to adapt to your unique circumstances.

Focus on:

Gradual Progression: Build your long run distance and overall volume patiently.
Individual Factors: Consider your experience, goals, time, race specifics, and injury history.
Quality over Quantity: Execute your long runs well – practice pacing, nutrition, hydration, and gear.
Consistency: Consistent training over months is more valuable than one heroic long run.
Specificity: Match your training terrain and conditions to the race as much as possible.
Recovery: Treat rest and recovery as integral parts of your training.
Listening to Your Body: Be flexible and prioritize staying healthy above all else.

Whether you opt for a peak single long run in the 50-70km range, leverage the power of back-to-back long runs, or focus predominantly on time-on-feet, the key is a structured, sensible approach. Your long runs are your dress rehearsals for the main event. Use them wisely to build the physical resilience, mental fortitude, and practical skills needed to conquer the incredible challenge of the 100k. Trust your training, respect the distance, and get ready for an unforgettable journey.