What Happens To Your Body When You Run A Marathon? 

A marathon is an incredible challenge for your body, and also your mind, and a lot of things happen during the course of 26.2 miles, or 42.2km.

Here’s everything you need to know about what happens to your body when you run a marathon – if you want to jump to the video version, then it’s at the bottom of the page. 

A Bad Night’s Sleep

Yep, we’re starting before the marathon even begins, and because of the nerves and excitement of race day you probably won’t get much sleep (an early morning alarm on race day won’t help). 

Do your best to sleep well in the week leading up to your marathon, and don’t worry too much about the night before. It’s unlikely to have a significant effect on your marathon because the adrenaline of race day will get you through (and some caffeine could help).

Get Psychologically Ready

You’re physically ready to run, but is your mind ready? The day before the race you want to be prepared for what’s coming, and that means eating and drinking well, packing your kit, and making plans for your race. Minimise the things you need to think about on race day because you need your brain to be focused on the run and not worrying about anything else. 

Let’s head to the start line. On your marks… 

Your Muscles Start Working & Your Heart Rate Increases

Hopefully you’ve done a warmup, which could be a little jog, or just some dynamic movements and drills, as this gets your body ready for running.

As you begin to run, your muscles start to work harder to move you forward. The primary running muscles are in the legs, but we also use a lot of muscles in the core and upper body, especially to stabilise us, which is why our lower back and abs can ache late into a long run.

Your heart rate and breathing will increase as you run and this is because you now have a greater need for oxygen. 

When you inhale air, your body absorbs oxygen into your bloodstream. Your heart pumps blood around your body to your muscles, delivering the oxygen which you need to create energy. Blood also carries other important things like energy in the form of glucose (more on that later!). 

When your blood cells give oxygen and glucose (in the form of glycogen) to the muscles, they also remove carbon dioxide and other waste products, like the lactate that can make our legs feel heavy and tired.

The harder you run, the more oxygen you need to breathe in. And the harder you work, the more you sweat…

You Start To Sweat

Exercise creates heat in the body, and we cool ourselves down by sweating (see more about the science of sweat here). As blood is sent to the muscles to give them oxygen and fuel, it’s also pumped towards the surface of the skin, where it passes sweat glands which remove water from the blood and send it to the skin. As sweat evaporates, we feel cooler.

An average person will lose 3-6 litres of sweat during a marathon. That’s a lot! Some people naturally sweat more than others, while hot weather will have us sweating more heavily than cold weather. Because we sweat, we need to rehydrate to avoid dehydration… 

We Need To Hydrate

Don’t drink to thirst, and instead focus on sipping little and often during the run. Aim for 500ml per hour, and more if you can tolerate it, to reduce the risk of dehydration (this is something you need to practise in training).

Most road races have water stations every 1-3 miles/2-5km (they may be further apart in trail races), or you could carry your own water in a soft flask, bottle or hydration vest if you prefer. 

Avoiding dehydration is very important. When we’re dehydrated, we have less blood volume so the heart has to work harder to pump enough blood around the body, which makes running feel so much harder. Other symptoms of dehydration include feeling nauseous, dizzy, weak and confused, and it could lead to more serious problems which may stop you from finishing your race.

But be careful: it’s possible to drink too much water, and a condition called hyponatremia can be extremely dangerous. This happens when we dilute our body’s electrolyte balance, so you should take electrolytes with water during a marathon. 

You Might Need To Use The Toilet

You’ve drunk a lot of water leading to the start line, and even though you used the toilet a few times before the gun went off, there’s a chance you’ll need to go again.

No matter how fast you hope to run, if you have to go, then it’s best to just stop. Taking one minute to use a portaloo on the course will save you from a lot more discomfort later in the race. 

We Use A Lot Of Energy, And We Need To Take On More

The body’s primary fuel source is glucose, or sugar, which we get from carbohydrates. We carb-load in the days before a race, meaning eating extra carbohydrates which our body stores as fuel. 

But like a car, we can only store a finite amount of fuel before our tank gets empty – and our body can’t physically store enough to last for a full marathon. When we run out of energy, we hit The Wall. 

‘Hitting The Wall’ is when our body no longer has glucose left for fuel, so it has to resort to using fat as the fuel, which is much harder to utilise (basically we need to use lots of oxygen to turn fat into energy, meaning most of the oxygen we breathe in has to be used for that, and our body slows down).

To help avoid The Wall, we need to be taking on energy while we run. And we should start doing it early and often. 

Most people choose energy gels, energy chews and energy drinks as they are the most efficient fuels and the easiest to digest. You’ll want to aim to take in 50-90g carbs per hour (around two gels plus energy drink) – but you’ll want to practise doing this in training to know what energy works best for you, and how much you can take in without causing stomach problems. 

The Middle Miles

There’s an old saying that ‘A marathon begins at mile 20’ (or 30km for metric folk). It means that if you’ve trained right, then getting to 20 miles should be ok, but the final six (the last 10km) are going to be tough as your body is tired, you’re dehydrated, you risk Hitting The Wall, and everything just generally hurts. 

That saying is pretty accurate, but you still need to actually run those first 20 miles.

You should aim to maintain a steady pace. If we want to run a four-hour marathon, then that’s just over 9 minutes per mile (5:40 per km). You don’t want to be speeding up and slowing down, as that can add extra strain on your body. If you can be consistent then your body can settle into the pace, and your heart rate and breathing can remain constant, which is very important in the marathon. 

When you get to halfway, it’s much better if it feels too easy than too difficult. You can always choose to speed up later, whereas slowing down is never something you choose to do – it’s something you have to do because your body can’t carry on at the same pace. 

In these middle miles it’s important to not get overwhelmed…  

Don’t Focus On The Whole Distance, And Break It Down

While it’s natural to count down the distance that’s left in the race, that can come to feel overwhelming and extra challenging as the run gets harder. You might immediately feel happy to see mile marker 16, but knowing you’ve still got 10 miles to go can feel like a long way. 

Break the race down into 1km or 1 mile chunks, and focus on maintaining a good pace for just that one block of distance.  

If you’re aiming to run sub-four hours, then if you run each mile in 9:05, or each kilometre close to 5:40, then you’ll get your goal. Take it one mile or kilometre at a time. 

You May Feel Sick

This one sucks. When we run, our body prioritises getting blood to our muscles to keep us going, and that means it deprioritises other parts of the body, including the stomach. So when we try to consume liquid and energy gels (which we have to do to have enough energy), our stomach can struggle to deal with it, which can make us feel ill.

Starting a race well hydrated, including having electrolytes, can help. As can practising fueling during training to work out what is best for you.

And if you do feel ill then slowing down is probably the only way to help you feel better. 

Blisters and Chafing

Hopefully you’ll have practised wearing all of your kit in long runs so you know before a race whether anything might cause you any problems or discomfort, or you know to use tape, plasters or anti-chafe on particular parts of your body.

But different weather conditions – more heat, rain, humidity – or just the extra time it takes to run a marathon can cause issues you didn’t foresee. 

Sometimes you may be able to stop at a medical tent to get some help – a plaster or anti-chafe – or you might just need to carry on and finish the run and worry about it later. 

But if a blister or chafing changes the way you run then be careful as it could lead to injury somewhere else in your body. Deal quickly with anything that changes your stride.

Muscles Get Tired, So Focus On Your Form

You’ve got to mile 20 or 30km and you’re still maintaining the same pace you’ve been running for the whole marathon so far, but it feels so much harder now.

You’re having to put in more effort to maintain the same speed, you’re breathing more heavily, your heart rate is going up. 

Every step puts strain through your leg muscles. Over the duration of a marathon we’ll take something like 40,000 steps, and that damages and weakens the muscles, making it harder to run.

Your muscles also produce waste products during exercise and the body is usually able to clear these away, but later in a marathon that gets harder to do. The main one is lactate, which leaves your legs tired and heavy.

As you get tired and weak, you may start to run with bad form. You’ll slump over, your stride gets shorter, cadence gets slower. Focus on running with the best form you can. If it helps, then imagine a piece of string coming out of the top of your head and think about that being pulled and making you run really tall and upright.

And it’s not just our legs which get tired…  

We Can’t Think Straight, Or Start Having Negative Thoughts 

As we get more tired and dehydrated, our thought processes become more limited, and can get more negative or confused, which makes it harder to make good decisions. Our brain uses around 20% of all the energy in our body, so we aren’t just having gels for our legs, it’s for our head as well. 

You have to fight against the negative mindset, because while you can’t do much about aching legs, you can use your mind to help you. Work on having a race mantra, or sayings which can help get you through if you start to feel negative thoughts. Be positive!

Here Come The Cramps

You may start to feel cramping. It could be a deep ache that makes it hard to run properly, or it could be the dramatic type of sudden cramping where the muscle feels like it’s seized up.

Most cramping comes from general fatigue and it’s impossible to stop a cramp if it’s going to get you, but racing at a sensible pace can help you avoid (or at least delay) them. If you feel a cramp coming, then sometimes the best (or only) thing to do is stop and stretch. Being well-fueled and hydrated can help here. 

The Final Mile

Whether you’re chasing a PB or just trying to finish, the last mile always hurts. It feels further than any mile you’ve run before. Your heart rate might be nearing its maximum. The pains are at their worst. You just want this to be over. 

But the crowd support is also some of the best. Take in those cheers. Make sure you soak up the support as it can be a big boost in those final few minutes. 

As you get to the finish line, something magical happens and all the discomfort disappears for a few wonderful seconds.

It won’t last long, so enjoy it while you can…

After The Race

The finish line feeling is an enormous mix of emotions. Many people start crying as soon as they cross the finish line. There’s joy and relief. Pride. Sometimes there’s sadness or disappointment at not achieving a goal. 

As you stop running and start walking, your legs will feel unstable and weak. Your lungs ache from the effort. Your heart rate will remain high, but will gradually come down.

If you sit down, you might feel muscle cramps come on (and it’s hard to get back up again!).

You’ll be thirsty and will need to drink, but you may also struggle to eat for an hour or two after a marathon as your stomach settles (or you may be ravenous immediately!). 

Focus on prioritising your recovery. You’ll have aches, you may be extra tired or extra hungry, you may feel ill as your immune system is weakened, and you may feel sad that the marathon is over. Just look after yourself. 

Most of all, celebrate that you’ve done something amazing and you’ve run a marathon! 

Here’s Sarah and Mo telling you all about what happens to your body during a marathon.