The 100-Miler Execution Guide

Most 100-mile DNFs are not a fitness problem. The runner who drops at mile 70 usually trained just as hard as the runner who finishes. They went out too fast, stopped eating too early, or drank too much without enough sodium. This guide covers the four things that decide whether you finish: effort, carbs, electrolytes, and hydration. Each one has a clear target. Know yours before the start line.

Effort, Pace, and Heart Rate

GPS pace is useless in a 100-miler. A 14-minute mile on flat road and a 14-minute mile up a steep climb are completely different amounts of work. Run by heart rate or feel, not by speed.

The first half: protect everything

The goal in the first 50 miles is to reach mile 50 feeling like you have more left. That means a conversational pace: you can speak full sentences without gasping. On a heart rate monitor, that's about 60 to 75% of your max heart rate on flat ground. On steep climbs, walk at that same effort level. Most coaches and experienced ultra runners follow one rule: if the grade raises your heart rate, walk it.

Coates et al. (2021) studied 59 runners in 100-mile races. They found that none of the standard fitness measurements predicted who would finish fastest. Not VO2 max. Not lactate threshold. Not running economy. What mattered was keeping forward movement in the second half. Runners who went hard early ran out of runway.¹

Tiller and Millet (2025) explain why. Past mile 60 or so, the main thing that slows you down is not your lungs or your heart. It is leg muscle damage from running downhills. Every hard descent in the first half makes the second half harder. Run easy on the way up. Run carefully on the way down.²

Heart rate targets by terrain

Terrain	Miles 0–50	Miles 50–80	Miles 80–100
Flat runnable	60–70% max HR	65–75% max HR	Run what you have
Moderate climb	65–75% max HR	70–78% max HR	Run what you have
Steep climb (>15%)	Walk. Stay below 78%	Walk. Stay below 80%	Walk. Stay moving.
Descent	Controlled. No pounding.	Controlled. Protect quads.	Manage what's left.

These numbers require knowing your max heart rate. If you don't know it, use feel. Zone 2 is a pace you could hold for hours. You can talk, but it takes a little effort. Zone 3 and above is where talking gets hard. That zone is fine for short pushes, but you cannot live there for 100 miles.

The hiking rule

Walking is not failure. Hiking steep climbs is the smart move. It uses different muscles than running and gives your legs a partial break. On grades steeper than 10 to 15%, hiking is often faster than trying to run anyway. Elite 100-mile runners hike a lot. The difference between their hike and a casual shuffle is intention: arms driving, core tight, moving at 3.5 to 4.5 mph instead of 2.

Carbohydrate Intake

Carbs are your main fuel. Your body burns both fat and carbs during a 100-miler, but carbs power your pace and keep your brain sharp late in the race. Run out of carbs and you hit the bonk: irrational thoughts, the urge to quit, legs that feel okay but won't move. It is one of the worst feelings in the sport and it is almost entirely preventable.

The target: 60 to 90 grams per hour

Research puts the target at 60 to 90 grams of carbs per hour.³ In the early miles when you are moving faster, aim for 80 to 90 grams. In hiking sections and overnight stretches, 60 to 70 grams is enough and easier on your stomach.

What does 60 grams look like? Two energy gels (about 22 to 25 grams each), or one gel plus a banana and a handful of pretzels. Eat something every 20 to 30 minutes. If you are waiting until you feel hungry, you are already behind.

Finishers eat more than DNFs. A study of the Western States 100 found that finishers took in roughly 70 grams of carbs per hour. Runners who dropped averaged less than 45 grams. Same race, same trails. The difference was not fitness. It was food.⁴

Why 120 grams per hour backfires

Some runners chase 120 grams per hour after seeing data from shorter races. A 2025 study by Hearris et al. tested what actually happens at that level. At 120 grams per hour, the gut only converted about 75% of that carb into usable fuel. At 90 grams per hour, it converted 86%. The extra 30 grams per hour did not help. It sat in the gut, pulled in water, and caused stomach problems.⁵ More carbs did not mean more fuel. It meant more nausea.

Ultras are longer and slower than marathons. Your gut has less blood flow and less tolerance for big volumes of food. Eat consistently at 60 to 90 grams and you will have more fuel available than the runner who tries to cram in 120.

Glucose-fructose blends

Your gut absorbs glucose and fructose through two different pathways. If you only eat glucose, one pathway gets maxed out at about 60 grams per hour. Add fructose and you open a second pathway, which lets you absorb up to 90 grams per hour without as much stomach trouble. Look for gels and chews with a 2-to-1 mix of glucose to fructose. If the first two ingredients are maltodextrin (or glucose syrup) and fructose, you have the right blend. Pure maltodextrin gels max out around 60 grams per hour.

Real food in the back half

After mile 50 or 60, gels get hard to stomach. The sweetness becomes too much. The texture feels wrong. This is normal. Aid stations at major 100s are stocked with real food for exactly this reason: broth, boiled potatoes, rice, ramen, watermelon, boiled eggs. These are not just comfort food. They are fuel. Potatoes with salt give you fast carbs and sodium in a form your gut can handle when gels cannot. Broth gives you sodium and warmth in the middle of the night. Rice with salt is easy to digest. Know which aid stations have what before race day and plan your shift from gels to real food.

Electrolytes

Electrolytes matter as much as carbs. Sodium controls how your body holds and moves fluid. Too little sodium can cause hyponatremia, a dangerous condition where your blood becomes too diluted with water. Too little sodium while you are sweating hard leads to cramping, confusion, and a general breakdown of how your cells work.

Sodium: the primary target

The International Society of Sports Nutrition recommends 500 to 1,000 milligrams of sodium per hour for ultra runners, adjusted for heat and sweat rate.³ That range is wide because people vary a lot. Some runners lose only 200 mg of sodium per liter of sweat. Others lose more than 2,000 mg. If your shirts and hats have white salt rings after long runs, you are a salty sweater and should aim for the high end in hot weather. Lighter sweaters in cool conditions can go lower.

Condition	Sodium target/hr	Source examples
Cool weather, moderate sweat	500–700mg	1 electrolyte tab + salted food
Hot or humid, moderate sweat	700–900mg	2 electrolyte tabs or 1 tab + broth
Hot, heavy sweater	900–1,200mg	2 tabs + salted potatoes + broth
Overnight, cold, low intensity	400–600mg	1 tab or broth every hour

Common sodium sources at 100-mile aid stations: electrolyte tabs (250 to 500 mg each), broth (about 800 to 1,000 mg per 8 oz cup), boiled potatoes with salt (variable), pretzels (about 200 mg per small handful), and pickle juice (about 300 mg per 2 oz shot).

Potassium, magnesium, and calcium

These three matter, but less than sodium. Potassium and magnesium are often blamed for cramps, but the research is mixed. Cramps in ultras are usually caused by tired muscles, not low electrolytes. That said, eating real foods like bananas, oranges, and potatoes covers most of what you need. An electrolyte tab that includes magnesium takes care of the rest.

Hydration

Drink when you are thirsty. Most runners ignore this because they think they need to stay ahead of thirst to perform well. That is not true and in ultras it can be dangerous.

Why drinking to thirst works

Thirst is a built-in signal. It kicks in before you are meaningfully dehydrated and tracks your actual fluid needs pretty closely. Noakes and colleagues studied this across decades of ultra endurance events including Comrades and Ironman. They found that the performance drop from mild dehydration is much smaller than people assume, and that drinking to thirst works better than drinking on a schedule while also preventing hyponatremia.⁶

Heat is the one exception. In hot conditions, thirst can lag a little behind your actual need, especially in the first hour or two. In that case, take a few sips every 20 minutes even if you are not thirsty yet. But "drink before you are thirsty" does not mean "drink as much as you can at every aid station." It means stay slightly ahead, not flood your system.

How much is too much

Check your urine color. Clear and frequent means you are drinking too much. Light yellow is the target. Dark yellow means you need more. No output for several hours in hot weather is a warning sign. If you feel bloated and nauseous but not thirsty, stop drinking and take sodium. That is usually hyponatremia, not dehydration.

For rough numbers, most runners need 16 to 24 ounces per hour in moderate weather, and 24 to 32 ounces in heat. But these are just estimates. Thirst is more accurate than any formula.

What to drink

Water is the main drink. Sports drinks have carbs and fluid together, but the carb concentration (usually 6 to 8%) limits how much you can take in before your stomach slows down. At high volumes, sports drinks cause stomach problems. Most experienced ultra runners drink water and eat their carbs as solid food or gels. Broth at night does two jobs at once: sodium and warmth.

Altitude and heat adjustments

Above 8,000 feet, you lose water through your breathing. Cold, dry air at altitude dries out your airways fast. Runners at Leadville, Hardrock, or Bighorn need to drink a little more proactively because this loss does not trigger thirst the same way sweating does. On the other hand, altitude means you go slower for the same effort, so you sweat less. It roughly evens out. The adjustment: drink a bit more water than you think you need, and do not cut your sodium.

Putting It Together: A Simple Race-Day Framework

These four rules work together. Get all four right and you give yourself the best shot no matter your fitness level. Get one or two right and you will probably finish, with more suffering than necessary. Ignore all four and even a well-trained runner is one bad patch away from a DNF.

Practice this in training. Every long run over 90 minutes is a chance to eat on a schedule, test products, and figure out your sweat and sodium needs. Race day is not the time to find out your stomach hates the gels at the aid stations.