How to Pace a 100-Mile Race

Your GPS watch is going to lie to you. At mile 4 of a 100-mile ultramarathon, a 14-minute mile might be a hard effort. At mile 14, that same 14-minute mile might feel easy. At mile 74, on a descent your quads no longer want to run, it might be everything you have. Pace-per-mile is a metric built for flat roads, and it tells you almost nothing useful across 100 miles of variable terrain.

The runners who figure this out before race day tend to finish. The ones who do not tend to die in the back half. And according to research following ultramarathon runners across distances from 50K to 160 kilometers, pacing discipline is one of only three variables that consistently separates strong finishers from people who slow to a walk at mile 70.¹

Why Pace Does Not Work

In a road marathon, pace is a reliable proxy for effort. The terrain is consistent, the conditions are predictable, and heart rate at a given pace is roughly stable across the race. You can build a split sheet in minutes and follow it.

A 100-mile ultramarathon has none of those properties. A 10-minute mile on flat trail and a 10-minute mile up a 20% grade are completely different efforts. The same nominal pace can feel like a jog at mile 5 and feel like a sprint at mile 75, when your glycogen is depleted, your quads are damaged from 20,000 feet of descent, and you have been moving for 22 hours. Chasing a pace target in that context does not keep your effort steady. It just makes your splits look good while your race falls apart.

The alternative is effort-based pacing: using heart rate, perceived exertion, or breathing as the primary signal, and letting pace be whatever pace those targets produce on a given segment.

The Only Number That Matters in the First 50

Most experienced 100-mile runners will tell you some version of the same thing: the first 50 miles of a 100-miler are run with your brain, not your legs. The physiological consequences of going out too hard do not show up immediately. You feel fine at mile 20. You feel fine at mile 30. The penalty collects silently, and presents itself somewhere between mile 60 and mile 80 as a pace collapse you cannot run your way out of.

The specific mechanism is twofold. Going out too hard depletes glycogen faster than you can replace it, compounding the difficulty of fueling in the later miles when appetite fades. It also accelerates muscular damage on the descents, where an overly aggressive pace forces your quads to absorb more eccentric load per mile than a conservative pace would. You arrive at the back half of the race both underfueled and with legs that have already taken more damage than they can fully recover from in motion.

The target for the first half of a 100-miler is an effort that feels almost embarrassingly easy. Heart rate in Zone 1 to low Zone 2 (roughly 50 to 65 percent of maximum) on runnable terrain. Walking any climb that would push you above that ceiling. Conversations should be possible. You should be passing aid stations thinking you could go faster, and choosing not to.

Effort Targets by Segment

A useful way to structure a 100-mile pacing plan is to break the race into five segments, each with a different primary goal. The distances below are illustrative; adjust based on your specific race's aid station layout and the location of major climbs.

The Walk Strategy

Walking in a 100-mile race is not a sign the race is going badly. It is a core part of pacing strategy, and the runners who use it deliberately tend to finish faster than the ones who avoid it until they have no choice.

The core walk rule is not actually about walking. It is about keeping effort below a ceiling on uphills, from mile one. For most runners, that means hiking any grade where jogging would push heart rate above low Zone 2. For runners with a strong aerobic base who can jog a moderate climb and stay in Zone 1 to 2, jogging is fine. The measure is effort, not motion. What matters is not running when your heart rate says you should not be running.

In practice, most runners will walk more than they expect to, especially on sustained climbs in the back half. The energy cost of running a steep grade versus hiking it briskly is significant; the time difference is usually minimal. On a 20-plus-hour day, the effort savings compound. Walking a climb at hour 18 that you could theoretically jog is an investment, not a concession.

Beyond climbs, strategic walking serves two other functions. It allows you to eat and drink without the coordination challenge of running, which matters more than it sounds when you are 20 hours in and fatigued. And it gives the quads brief recovery windows that compound across the back half of the race. Walking a flat mile at hour 22 feels like surrender. It often keeps the next five miles from becoming a shuffle.

Managing the Back Half

The miles from roughly 65 to 85 are where most 100-mile races end or fall apart. Heart rate data loses some of its utility here because cardiac drift, the gradual rise in heart rate at a fixed effort level over many hours, means your HR is no longer a clean proxy for intensity. Perceived effort and breathing become the more reliable signals.

The goal in this segment is not to run a specific pace or hit a specific HR zone. It is to keep moving with whatever effort you can sustain continuously. Continuous forward motion at a slow pace almost always produces a better finishing time than alternating running bursts with extended walking stops. If you are reduced to a walk, walk briskly. Four miles per hour of walking covers ground. A shuffling jog is sometimes slower.

Fueling in the back half requires active effort. Appetite usually fades significantly after 10 to 12 hours of racing, and many runners mistake the absence of hunger for not needing calories. The absence of hunger at hour 14 does not mean your body does not need fuel. It means your GI system is working hard enough that appetite signals are suppressed. Eat on a schedule, not on hunger. Broth, potatoes, and fruit often go down more easily than gels at this stage. The fueling article covers the specific targets in detail.

Building Your Race Day Plan

A good 100-mile race plan is not a split sheet with expected finish times at every aid station. It is a set of behavioral rules that you follow regardless of how you feel. Feelings are unreliable over 24-plus hours. Rules are not.

• Set a hard ceiling for the first 30 miles. If your watch shows you above low Zone 2 on flat runnable trail, slow down. The ceiling is not a target. It is a maximum you do not exceed, regardless of how good you feel.
• Define your uphill rule before race day. Walk any climb where jogging would push you above low Zone 2. Fit runners who can hold Zone 1 to 2 on a moderate grade can run it. The rule is about effort, not pace. Decide in advance so you are not negotiating with yourself at mile 8 when everything feels fine.
• Eat at every aid station. Even if you are not hungry. Even if you just ate. Caloric surplus early is insurance against the inevitable appetite suppression in the back half.
• Slow down at night. Running in the dark on technical terrain at race effort is how ankles get sprained. Drop your effort target 10 to 15 percent after sunset and recover it after sunrise if you still have the legs to run.
• Make decisions at aid stations, not on the trail. The trail at mile 70 is not a good place to problem-solve. Sit down, eat, assess, and leave with a clear plan for the next segment.

What Good Execution Actually Looks Like

A well-paced 100-mile race feels wrong for the first six hours. Everyone around you is running faster. Your watch is showing paces that feel slow for your fitness level. You are walking climbs that other runners are jogging. This is correct. This is the plan working.

The inversion typically happens somewhere between mile 50 and mile 70. The runners who went out hard are slowing. You are not. The relative pace difference between you and the field at mile 60 is a direct function of the gap at mile 10. Every mile you spent underrunning in the first third of the race buys you roughly that mile back in the final third.

The research backs this up. A 2021 study following trained ultramarathon runners found that standard fitness markers — VO2 max, lactate threshold, running economy — did not predict finishing time at 160 kilometers.¹ A separate 2025 survey of 824 ultramarathon finishers identified what does matter: pacing breakdown, fueling failure, and muscular fatigue ranked as the top race limiters across all distances surveyed.² You build the second and third of those in training. The first one is a decision you make at mile 4, when everything feels easy and the temptation to run faster is highest.