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. One study of trail runners found that lab fitness tests could not predict who did best in a 100-mile race.¹ Fitness gets you to the start. How you run the race matters a lot after that.

Why Doesn't Pace Work as a Strategy for 100-Mile Races?

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 pace can feel like a jog at mile 5 and a sprint at mile 75. By then your fuel stores are empty, your quads are wrecked from 20,000 feet of downhill, and you've been moving for 22 hours. Chasing a pace then doesn't 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 seasoned 100-mile runners will tell you the same thing: you run the first 50 miles with your brain, not your legs. The cost of starting too fast doesn't show up right away. You feel fine at mile 20. You feel fine at mile 30. The bill builds up quietly. Then it comes due between mile 60 and mile 80, when your pace falls apart and you can't run your way out.

Two things go wrong. First, a fast start burns your stored carbs (glycogen) faster than you can replace them. That leaves a bigger hole to fill later, when you don't feel like eating. Second, it beats up your legs on the downhills. A fast pace makes your quads soak up more pounding every mile. You reach the back half low on fuel and with legs too damaged to recover while you're still moving.

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.

What Should Your Effort Feel Like at Each Stage of a 100-Miler?

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.

When and How Should You Walk During a 100-Mile Race?

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.

Most runners walk more than they expect, mostly on long climbs late in the race. Running a steep hill costs a lot more energy than hiking it fast. The time you save by running is usually small. Over a 20-plus-hour day, the saved energy adds up. Walking a climb at hour 18 that you could maybe jog is a smart trade.

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.

How Do You Manage Miles 50 to 100 of a 100-Miler?

Miles 65 to 85 are where most 100-mile races end or fall apart. Heart rate is less useful here. Over many hours, your heart rate slowly climbs even at the same effort. This is called cardiac drift. So your heart rate stops matching your effort as closely. How you feel and how you're breathing are better guides.

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.

Eating in the back half takes work. Most runners lose their appetite after 10 to 12 hours of racing. Many think that means they don't need food. Not being hungry at hour 14 does not mean your body doesn't need fuel. Your gut is so stressed that it stops sending hunger signals. 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 tested runners in a lab before 50K, 50-mile, and 100-mile trail races. Lab scores predicted the 50K and 50-mile times, but not the 100-mile times.¹ The study didn't look at pacing. But it does suggest that fitness alone won't carry you through a 100. In a poll of 824 ultrarunners, cited in a 2025 review, 66% said muscle damage held them back most. Only 23% said stomach trouble.² You can train your legs for the pounding. Pacing is a choice you make at mile 4, when everything feels easy and you want to run faster.