Carb Loading for London Marathon Race Day: Your Pre-Marathon Timeline

The marathon run training block is almost behind you. Weeks of early mornings, long runs, and sore legs have brought you to this point. Now your job changes.

The final 72 hours before running the London marathon aren't about fitness, that's done. They're about showing up to the start line with full glycogen stores, a calm gut, and a plan you've already practised.

Here's everything you need to know.

Carb loading is not just an excuse for a big pasta dinner. It is a deliberate, evidence-based strategy to maximise the glycogen stored in your muscles and liver before race day.

As we covered in Part One, your glycogen stores are your primary fuel source during a marathon. They deplete within 90 minutes at race intensity, and once they're gone, the wall arrives. Carb loading extends that ceiling (Cao et al., 2023).

The research-backed target is 10–12g of carbohydrates per kg of body weight per day in the 2 days before your race (Cao et al., 2023). For a 70kg runner, that's 700–840g of carbohydrates daily. Most runners significantly underestimate how intentional they need to be to hit this.

Most runners significantly underestimate how intentional they need to be to hit this, which is why following a structured marathon runner meal plan can make the difference between feeling prepared and hitting the wall.

Three things shift simultaneously from 72 hours out:

Movement: Taper your step count. A light shake-out run is fine, but your fitness will not improve this week. Your energy stores can.

Sleep: Aim for 8–9 hours per night. Sleeping two nights before the race matters more than most runners realise. Race-day nerves often disrupt the night before, so prioritise sleep in the week leading up to your race.

Hydration: Keep fluid intake steady. Your body requires additional water to store glycogen - roughly 3g of water is held alongside every gram stored. Stay consistently hydrated to support this process (Murray and Rosenbloom, 2018).

The most common mistake is eating more food overall and arriving at the start line bloated and heavy.

The strategy is not to eat more, it's to redistribute. Reduce fat, fibre, and protein to make room for carbohydrates, keeping total calories roughly the same. Here's the marathon nutrition plan:

If solid food becomes difficult, liquid carbohydrates are your friend. A 300ml glass of fruit juice delivers approximately 25–30g of carbohydrates without the fullness of a full meal. Eat little and often throughout the day rather than relying on large meals.

If you didn't eat it during your training block, do not eat it now. This is not the week for new restaurants, unfamiliar energy bars, or well-meaning expo samples.

Your healthy gut has been trained alongside your legs. Protect that. We want your legs to be the only thing working hard on race day, not your digestion.

Lunch is actually your most important meal the day before the race. Eating your largest carbohydrate meal at midday gives your digestive system maximum time to process it, this means you don't need a large dinner to hit your targets.

Dinner should be simple, familiar, and moderate: a bowl of white rice, plain pasta, or a large baked potato. Nothing rich, heavy, or unfamiliar. Finish eating by early evening so your body has had adequate time to digest before sleep.

Leave yourself enough time. A rushed morning is the enemy of a good race.

1.5–2 hours before you start, eat a familiar, carbohydrate-dense breakfast your gut has processed many times before:

If nerves make eating difficult, a large glass of fruit juice alongside something small is better than nothing. The gut-brain connection is real, and so pre-race anxiety genuinely affects digestion, which is exactly why practising your race-day breakfast throughout training matters so much.

30 minutes before the start, keep it fast-acting and light: a few jelly babies, a couple of dates, or half a gel. A small hit of simple sugar without anything sitting heavily on your stomach.

The training is done. Race day is the celebration of the work. You're ready!

The final days before race day aren’t the time to second-guess your nutrition. With the Calo App, your meals are already taken care of, so you can focus on executing your plan. From balanced dishes to carb-focused, high-protein options, and more, everything is designed with the right portions and macros to support your training.

If you’re looking for healthy halal food during race week, simplicity is key. With Calo, you get convenient halal meal delivery that fits seamlessly into your routine, no prep, no stress.

At this stage, there’s nothing left to build, only to protect and execute. Your fitness is already there. What matters now is showing up on race day with full glycogen stores, a settled gut, and a plan you trust. Avoid last-minute changes and focus on the controllables; your nutrition, your pacing, and your mindset.

Be intentional with every step. Stick to familiar foods, prioritise carbohydrates, and keep everything consistent with what you’ve practised. The same applies to your race morning: simple, predictable, and stress-free.

You’ve done the hard work. Now it’s about putting yourself in the best position to enjoy the London Marathon and cross that finish line strong.

Read about marathon preparation tips & tricks and more about London marathon nutrition here, or explore more health-related topics on the Calo blog.

References:

1- Cao, W., He, Y., Fu, R., Chen, Y., Yu, J. and He, Z. (2023) ‘A review of carbohydrate supplementation approaches and strategies for optimising performance in elite long-distance endurance’, Nutrients, 15(17), p. 3905.

2- Murray, B. and Rosenbloom, C. (2018) ‘Fundamentals of glycogen metabolism for coaches and athletes’, Nutrition Reviews, 76(4), pp. 243–259.