Inspired by the London Marathon? Here are some top tips to help you run 26.2 miles!

Published on 25 Apr 2025

6:30 min estimated read time

Dr Andy Galbraith, Associate Professor of Exercise Physiology at the University of East London.

Every year, the London Marathon inspires people to get running, from total beginners lacing up a running shoe for the first time, lapsed runners digging out their old running shoes, to regular parkrunners deciding this is the year they will have a go at a marathon. The marathon is the ultimate running challenge, but every year thousands of people take on one for the first time, so why not have a go? The marathon is a race, but it's about endurance rather than speed, so what are the factors that affect your ability to run for several hours, and how do you need to fuel properly when training and before, during and after the marathon? There can be a lot of jargon involved if you are new to sports training, but don't worry, I’ll explain it as I go along.

Fitness

The physiological factors that affect your endurance are your VO2 max, lactate threshold and running economy. VO2 max is a measure of your maximum rate of aerobic energy expenditure, essentially how much oxygen your body can take in, transport and use during intense exercise. Think of it like a car engine, VO2 max tells you the size of your aerobic engine, and a powerful aerobic engine is important if you want to exercise continuously for a number of hours. We measure this in our laboratory with an ~8-12-minute test on the treadmill where we analyse your breathing through gas analysis, which gives us your VO2 max values in terms of litres per minute, scaled to your whole-body weight in units of mL.kg-1. min-1.

The average man will record scores between 44-50 mL.kg-1. min-1 for 20-24 year olds to 29-32 mL.kg-1. min-1 for 60–65-year-olds, while the averages for women range from 37-41 mL.kg-1. min-1 to 22-24 for the same age ranges. While your score may decline with age, the fittest 60-year-olds can record scores similar to the average 30-year-old, so it’s never too late!

Training will improve your VO2 max, with a combination of long-distance runs to build a strong base of aerobic fitness and high-intensity efforts to force your body to adapt to the demands of high-intensity exercise.

Lactate threshold is another important measure. During exercise, our body breaks down glucose to create energy. Lactate and hydrogen ions are produced as a part of this process. When running at slower speeds, we can remove these by-products using lactate as an energy source, but at faster speeds, these by-products increase in our blood up to your lactate threshold, where lactate accumulates in the blood faster than it can be removed.

We assess this in our lab with a treadmill test with 3–4-minute stages over ~25 minutes with a fingertip blood sample after each stage. The results give us two scores, which tell us the running speeds and heart rates best for recovery, steady and tempo training, so you can follow a training plan.

Running economy is another measure of your endurance and looks at how much oxygen you consume at a given running speed, and therefore how much energy you need. Using our car analogy again, whilst VO2 max is the size of your engine, running economy is how efficiently you use fuel (oxygen in this case), with more efficient runners using less oxygen than runners with poor economy at the same speed – a bit like how many miles per gallon you can get in a car. This figure can vary between runners with similar VO2 scores, so it is a useful test of your endurance performance. This is also measured with a multi-stage treadmill test over 25 minutes, where we analyse your breathing.

Food

Having worked out how powerful your engine is and how to improve it, you need to look at the fuel you are putting in it. The food you eat before, during and after training and racing provides the macronutrients – carbohydrate, protein and fat – that you need.

Long-distance running events rely on the availability of carbohydrate and fat as fuels. At the high exercise intensity you will hit during a marathon (possibly around 80 per cent VO2 max), carbohydrate will be the main fuel, which we store in the body in the form of glycogen, mainly in muscles. Fatigue and tiredness during a marathon can often be related to running out of this muscle glycogen.

You’ve probably heard of runners ‘carbo-loading’ before an event – eating a lot more carbohydrates for two days before a long event such as a marathon.

You also need to consume carbohydrates during the race. As carbohydrate is the main fuel during marathon running, your body’s stores of muscle glycogen will gradually deplete during the race. You usually have enough muscle glycogen stores to sustain endurance performance for around 90 minutes – after that, you will need to top up your stores. For most athletes, that will mean taking in around 60g of carbohydrate per hour, commonly in the form of a drink or a gel.

Protein is not just for bodybuilders; it plays an important role in how your muscles adapt and recover. Research suggests that long-distance runners and other endurance athletes will need more protein than the general population. If you run for about 30 minutes four or five times a week, you should eat about 1-1.2 g of protein for each kilogram of your body weight every day. This goes up to 1.2-1.5 g per kg a day if you’re doing around an hour a day, and around 1.6-2.0 g per kg for highly trained endurance runners and those performing regular (>3 times per week) strength training.

Fat is also crucial for several processes in the body; cell membrane structure, brain health, absorption of fat-soluble vitamins (A, D, E, K), hormone regulation and as a source of energy at certain exercise intensities

Fat intake would typically make up ~25-35 per cent of a distance runner's total energy intake, but high-fat, low-carbohydrate diets (ketogenic) may compromise performance in the majority of training sessions.

Refuelling after exercise is important to aid recovery and support adaptation to training in the two hours afterwards. Runners should consume plenty of carbohydrate (around 1.2g per kg) and protein (0.25-0.30g per kg).

Drink

Hydration is a key part of your fuelling. You will lose fluid through sweat during an event, depending on how long it lasts, how intense it is, the temperature and humidity, your body size and fitness level.

You can measure how much fluid you lose by weighing yourself before and after exercise, and how much fluid you drink during it. For example, if you weigh 80kg before exercise and 78kg after, and drank 500ml (0.5kg) during two hours of exercise, your total sweat loss would be 2.5kg, giving you a sweat rate of 2.5kg/ 2 hrs, which equates to 1.25l per hour.

Don’t try to drink more fluid than recommended, as it will not provide a performance advantage and in extreme cases risks hyponatraemia (low blood sodium levels), which can be dangerous and drinking when you are thirsty may be a suitable strategy.

Whilst fluid intake varies considerably between individuals, and therefore specific advice here is challenging, suggestions from research are that you should aim for 450- 750ml/hr (or 150- 250ml every 20 minutes) to be properly hydrated during a race.

Dr Andy Galbraith is Associate Professor of Exercise Physiology at the University of East London. He is a keen runner, having competed over middle-distance as a Junior. He is now a regular marathon runner. Andy has 15 years' experience providing exercise physiology support to a range of athletes, including Olympic, World Championship and Commonwealth medallists.

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