From cramps, to dizziness, to dehydration, running a marathon can push you to the limit.
But what happens to your body during the race? And how does it cope with 26 miles of brutal punishment?
We asked Charlie Pedlar, a physiologist at the English Institute of Sport, to explain.
RUNNING ON EMPTY
The body's main problem is that it's limited by fuel. When you 'hit the wall' you're running on empty.
Once you get to the 18-mile mark (or thereabouts) your glycogen stores are depleted and you're having to draw on 'survival' energy - eating away at your body's protein and fat.
You can replenish these by taking on water and energy drinks, but that may only have a limited effect.
Your body doesn't like to digest foods or absorb fluids at the same time as doing exercise. In a marathon you're asking it to do just that.
THE DREADED DEHYDRATION
The body loses fluid in two main ways - sweating and breathing.
A good few hours of heavy breathing will result in a lot of lost fluid, but the amount lost through perspiration varies greatly as some people sweat much more than others.
An above average runner doing a three-hour marathon could expect to lose 3-5 kilograms of weight. For every kilogram you lose, you need to drink 1.5 litres of water to replace it.
And it's not just water. Sweat also contains salt (mainly sodium and potassium) which helps the body absorb water. Over a long race like the marathon, you can sometimes see (or taste) a fine layer of salt forming on the skin.
The best way to gauge how much fluid you've lost is to weigh yourself before and after a race - preferably naked, or with sweat soaked clothing removed.
During exercise your muscles experience micro-damage - tiny, undetectable tears caused by constant pounding against the ground.
As you continue running, the body tries to support its tissues in the muscle and prevent further damage.
The body is saying: 'don't do this to me!'. As it tries to repair the damage, a small amount of swelling can occur.
As if that wasn't enough, there's always the risk of cramp.
Cramp occurs when fatigue and dehydration cause the muscle to tighten up and become incredibly sore. If you've prepared well and stay well hydrated, the risk of suffering is reduced.
FEEL THE FORCE
One issue that's quite unique to running is the impact on the body.
Every step you take is the equivalent force of 2-3 bodyweights, and it's estimated you'll finish the race two centimetres shorter than you were at the start.
And the more tired you get, the more you feel that force.
Pounding the streets makes you a little bit shorter
At average marathon pace, each foot spends around 200 milliseconds on the ground and 500 milliseconds in the air.
As fatigue sets in, your stride becomes less efficient and your foot spends more time on the ground.
And the longer it spends on the ground, the more time it spends absorbing all the force - and the more likely you are to suffer aches and pains in your feet, knees and hips.
A GOOD DAZE WORK
People who've run the entire race often feel light-headed when they finish the marathon - some very occasionally faint.
While you're running, you're making use of the body-muscle pump - the contraction of your muscles is helping the body return blood to the heart.
When you stop running, the body muscle pump effect is reduced and the body must readjust. This sometimes causes the runner to feel light-headed.
REPAIRING THE DAMAGE
The most important thing once you've crossed the line is replenish your body's fuel supply.
As soon as you finish you'll be given a sandwich. Eat it, and try and grab whatever snacks you can. Take on plenty of fluid as well, but make sure you steer clear of alcohol.
People not used to running long distances can have trouble walking properly for several days after a marathon due to muscle soreness. A few stretches will make it easier to bear
Some people opt for a light massage immediately after the race, to try and remove some of the waste metabolites floating around in the muscle.
But remember, after a marathon the muscles will be in a state of turmoil and a massage that is too severe at this time may damage them further.
The best option is to let them heal - go home, put your feet up and make sure everyone knows about your body's heroic struggle.