How to Fuel for a Long Run: The Practical Guide to Marathon Nutrition

Ask any experienced marathon runner what went wrong in a race that unravelled and the answer is usually one of two things: they went out too fast, or they did not fuel adequately. Often both. The two problems are related; poor fuelling accelerates the consequences of even modest early pacing errors.

Getting nutrition right in the marathon is not complicated, but it requires understanding, planning, and (critically) practice. Race day is not the time to experiment. By the time you stand on the start line, your fuelling strategy should be something you have rehearsed repeatedly in training.

Why Fuelling Matters Beyond 90 Minutes

The body stores carbohydrate as glycogen in the muscles and liver. These stores provide enough fuel for approximately 90 minutes to two hours of exercise at marathon pace. For runners targeting sub-3 hours, this means the stores are potentially adequate. For runners targeting three hours or beyond, the majority of recreational marathon runners, glycogen depletion is inevitable unless replaced during the race.

When glycogen stores deplete, the body is forced to rely more heavily on fat for fuel. Fat metabolism is significantly less efficient than carbohydrate metabolism at marathon pace, and the pace drop that follows glycogen depletion is what most runners know as 'hitting the wall'. It is not a mental barrier; it is a physiological one, and it can be largely prevented through appropriate fuelling.

How Much Carbohydrate Do You Need?

Current evidence supports targeting 30 to 60 grams of carbohydrate per hour for efforts lasting 60 to 150 minutes. For longer efforts, such as marathon and beyond, athletes who have trained their gut can often tolerate up to 90 grams per hour by using multiple carbohydrate sources (typically a combination of glucose and fructose, which are absorbed through different transporters in the gut).

For most recreational marathon runners, 45 to 60 grams per hour is a practical starting target. At 45 grams per hour, a four-hour marathon requires approximately 180 grams of carbohydrate taken during the race; roughly three to four standard energy gels per hour depending on the brand.

Use the Endurance Toolkit fuel calculator to estimate your specific requirements based on your goal time and bodyweight.

When to Start Fuelling

The most common error is waiting until you feel depleted before taking fuel. By the time hunger, fatigue, or reduced pace signals that glycogen is low, blood glucose has already dropped and performance is being compromised.

Begin fuelling within the first 45 minutes of a run lasting more than 90 minutes. This feels very early; the effort is comfortable, the legs are fresh, and eating feels unnecessary. That is precisely the right time to start. Taking carbohydrate while the gut is well-perfused and not under significant stress means absorption is efficient. Waiting until kilometre 25 means taking fuel when the gut is increasingly compromised.

What to Use: Gels, Chews, Drinks, and Real Food

Energy gels are the most common race-day fuel for marathon runners because of their practicality; small, portable, and quickly absorbed. Most gels contain 20 to 25 grams of carbohydrate. Isotonic gels can be taken without additional water; others require a mouthful of water to aid absorption and prevent gastrointestinal discomfort.

Sports drinks provide carbohydrate and electrolytes simultaneously, which simplifies the equation but can make it harder to control exact carbohydrate intake if aid station offerings vary. Carrying a mix of gels and taking sports drinks at aid stations when available is a practical approach.

Some athletes prefer chews, dates, banana pieces, or other real food options. These work, provided they are practiced in training, do not cause gastrointestinal issues during exercise, and can be managed practically at race pace.

Training the Gut

The gut adapts to taking on fuel during exercise, but that adaptation requires practice. An athlete who has never fuelled during training and then attempts 60 grams of carbohydrate per hour on race day risks significant gastrointestinal distress.

Practice fuelling on every long run during marathon preparation, not just occasionally. Use the same products you intend to race with. Identify which gel brand agrees with your stomach. Experiment with the timing. The gut training that happens across a 16-week marathon block is as important as the run training.

Key Takeaways

Marathon fuelling is a system, not an emergency measure. Start early, fuel consistently, use products you have tested in training, and target 45 to 60 grams of carbohydrate per hour. The runners who execute their nutrition plan with the same discipline as their pacing plan are the ones who still feel like athletes at kilometre 38.

Steve Barbour is a UK-based online running and triathlon coach. The Fuel Calculator on Endurance Toolkit will estimate your carbohydrate needs for any session length. Find it at endurancetoolkit.com/fuel-calculator.