Marathon Race Week Fueling Timeline: 72 Hours to the Start Line

You store roughly 500g of glycogen across muscle and liver, and a marathon at goal pace burns more than that. Nothing you swallow during the race closes the gap on its own. The 72 hours before the gun decide how much fuel you have when the clock starts.

This is a countdown, not a diet. The window opens three days out, the real work happens in the last 36 to 48 hours, and it closes about 20 minutes into the race with your first gel. Everything in between is timing.

72 hours out: start the load

The published target is 10 to 12g of carbohydrate per kilogram of body weight per day for 36 to 48 hours before an event lasting longer than 90 minutes. That comes from the joint position statement of the Academy of Nutrition and Dietetics, Dietitians of Canada, and ACSM (Thomas, Erdman & Burke, 2016), and it matches the IOC consensus review Burke and colleagues published in the Journal of Sports Sciences in 2011.

Most amateur marathoners should not use 10 to 12. That is the ceiling, built around athletes who have rehearsed it, and hitting it means eating nearly non-stop for two days. FuelCenter's carb loading calculator tiers the number by experience instead. For a 70 kg (154 lb) marathoner it returns:

First-timer: 7 g/kg, 489g per day, over a 36-hour window.
Intermediate: 8.5 g/kg, 594g per day, over 24-36 hours.
Experienced: 9 g/kg, 629g per day, over 24-48 hours.
Elite: 11 g/kg, 768g per day, over 24-48 hours.

Notice what 72 hours out is not. It is not day one of three maximum-carb days. Glycogen has a ceiling and you reach it fast. Bussau et al. (2002) fed eight trained athletes 10 g/kg per day while keeping them inactive and found muscle glycogen hit roughly 180 mmol/kg wet mass after a single day, then stayed flat across two more days of the same diet. You cannot bank extra by starting on Monday. You can only make yourself uncomfortable.

So 72 hours out is a ratio change, not a volume change. Carbs move to the center of every plate. Fiber and fat start coming down. You keep your normal calorie ballpark and change what fills it.

You will gain 2 to 4 lb, and that is the point. Water is stored with glycogen at about 3g of water per 1g of glycogen. Olsson and Saltin (1970) measured a 2.4 kg gain in body weight and a 2.2 L gain in total body water across a four-day load. If the scale does not move at all, you probably did not load. Runners who panic at that number on Saturday night and start cutting carbs are throwing away the entire week.

48 hours out: the low-fiber, low-fat shift

This is a plumbing problem, not a nutrition-quality problem. Fiber adds residue that is still moving through your gut on race morning, and fat slows gastric emptying, which means the carbs you are trying to store arrive late. For two days, the healthy version of a food is the wrong version.

White rice instead of brown. Sourdough or a plain bagel instead of seeded whole grain. Applesauce or juice instead of a whole apple. Peeled potatoes, pancakes with syrup, pretzels, honey, jam, gummy candy, low-fiber cereal, sports drink. Cut the salads, beans, bran, raw vegetables, cream sauces, and anything fried.

Liquid carbs do more work here than people expect. A 16 oz glass of apple juice is 55 to 60g of carbohydrate and takes 30 seconds. When your target is 594g and your appetite has quit on you, that is the difference between hitting the number and rounding down.

The taper makes this feel wrong. You are running half your normal volume and eating more than usual, which registers as sloth. It is not. Do it anyway.

One rule with no exceptions: nothing new. Race week is not the week to try a restaurant you have never been to, a gel brand you saw at the expo, or a fiber-heavy grain bowl someone recommended. Everything you eat in the last 48 hours should be something you have already tested on a long run or a normal training day.

24 hours out: the night before

The big pasta dinner is a ritual, not a strategy. Six hundred grams of carbohydrate is not going onto one plate at 8:00 PM, and trying to force it there is how people end up bloated at the start line with half the load undone.

Spread it. For a 70 kg intermediate runner at 594g, that looks like roughly 150g at breakfast, 150g at lunch, two snacks of 100g each, and a dinner closer to 120g. Five or six feedings, none of them heroic. The dinner is the smallest meaningful decision of the day.

Timing: finish dinner by about 7:00 PM. You want your stomach mostly empty when you go to bed and fully empty when the alarm goes off.

Size: normal. Not the largest meal you have eaten all week. If you are uncomfortably full lying in bed, you overshot.

What to avoid: cream and cheese sauces, large amounts of meat, spicy food, big salads, beans, and alcohol. One beer will not end your race, but it takes up room you need for carbs and it makes the sleep worse.

Then add a bedtime snack of 50 to 75g. Overnight you burn liver glycogen with nothing coming in, and a bowl of cereal or toast with honey before you turn the light off protects the store you spent two days building.

The 72-hour timeline

Carb targets shown for a 70 kg (154 lb) marathoner at the intermediate tier. Run your own numbers in the carb loading calculator.

Race morning: breakfast 3 to 4 hours before the gun

The guideline for a pre-event meal is 1 to 4 g/kg of body weight, 1 to 4 hours before exercise lasting more than an hour. For a marathon, take the long end of that window. Three to four hours gives the meal time to clear your stomach, and a stomach that is still working at mile 2 will not absorb the gels you need at mile 6.

For a 70 kg runner the range is 70 to 280g. FuelCenter defaults to the midpoint at 2 g/kg, or 140g, which is enough to matter and small enough to digest. Two plain bagels with jam is about 120g. Add 12 oz of sports drink and you are there. Three packets of instant oatmeal with honey and a banana gets you to roughly the same place.

Coffee: drink it. If you have caffeine every morning, race morning is the worst possible day to find out what withdrawal feels like. The ISSN position stand on caffeine puts the performance dose at 3 to 6 mg/kg, most commonly taken 60 minutes before exercise. For a 70 kg runner that is 210 to 420 mg, roughly two to four cups. Plan the bathroom consequences into your morning instead of pretending they will not happen.

Fluid with sodium: 16 to 24 oz in the two to three hours before the start, with electrolytes rather than plain water. Plain water on an empty stomach mostly passes through you; sodium is what makes you retain it. If you want an exact number for your weight and the forecast, the hydration calculator will give you one.

The last 60 minutes

The three-hour breakfast rule assumes you know when the gun goes off and that it is soon. At the big city marathons, it often is not.

The 2026 Boston Marathon ran six waves between 10:00 and 11:21 AM, with the last wave loading buses in Boston at 9:00 AM. NYC pushes its final waves past 11:00 AM after a security check on Staten Island hours earlier. A runner who eats at 6:00 AM and starts at 11:20 AM is five and a half hours removed from breakfast, which is not a pre-race meal anymore. It is yesterday.

Split it. Eat the real breakfast on your normal schedule relative to waking, then add a second, smaller feed of 30 to 50g about 90 minutes before your wave. A banana, a bar, a bottle of sports drink. Something you can carry through the corrals and throw away.

Then take a gel or a serving of chews 15 minutes before the gun. 25 to 40g, chased with a few sips of water. This is the single most skipped step in race-day fueling, usually because someone was told that carbs close to the start cause a blood sugar crash.

That warning does not survive the evidence. Jeukendrup and Killer (2010) reviewed the pre-exercise carbohydrate literature and concluded that the advice to avoid carbs in the hour before exercise is unfounded. A transient dip in blood glucose does happen in some people, and it normalizes within roughly 10 minutes of starting to run, with performance unaltered or improved in nearly every study they examined. Taking the gel at 15 minutes sidesteps the 30-to-60-minute window anyway.

Sip, do not chug. Fluid in the last hour goes down 4 to 6 oz at a time. Draining a 20 oz bottle in the corral gives you a sloshing stomach and one more port-a-potty stop, not hydration. Whatever you have not absorbed by the time you cross the mat, you are carrying.

The first 30 minutes: spending what you built

Take your first in-race gel at 20 to 25 minutes. Not at the first aid station you happen to notice, and definitely not when you start feeling like you need one. By the time you feel it, you are 20 minutes behind.

The reason is absorption lag. A gel takes 15 to 20 minutes to reach your bloodstream, so one taken at 22 minutes is doing work at 40 minutes, which is exactly when the pre-start gel is fading. That handoff is the whole game. Miss it once and every gel after it is chasing a deficit instead of holding a line.

From there you settle into your cadence: 75 to 90g of carbs per hour for most runners, and 100+ if you have trained your gut for it. What that looks like in practice depends on your finish time and your gel brand, and that math is worth doing before race week rather than during it. The fueling plan by finish time breaks it down by goal, and the calculator will build the minute-by-minute version for whatever you actually carry.

Individual tolerance varies, and it varies a lot. Some runners handle 120 g/hr and a 300g breakfast without noticing; some get 60 g/hr and a plain bagel and that is the honest ceiling. Find yours in training, then execute the number you found rather than the number you read.

The carb load sets the size of the tank. The race spends it. Nothing you decide at mile 20 changes either one.

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