But what exactly does your body go through once the starting gun fires? Here’s a mile-by-mile breakdown of what happens when you run a marathon.

The First 5 Miles
The excitement of a crowd means most runners start fast. That’s a problem, says Siobhan M. Statuta, MD, a sports primary care specialist at UVA Family in Charlottesville, Virginia, because you’ll end up burning through your body’s energy stores earlier and fatiguing faster. Not great when you still have (gulp) more than 20 miles to go. “Slow and steady gets the job done,” Dr. Statuta says.

Unlike shorter running events (like a 5K, or 3.1 miles), where you don’t have to worry much about food or water, longer distances mean increased sweat and a greater calorie burn because you’re out there longer, according to Mark A. Harrast, MD, a sports physician at UW and medical director of the Sports Center at Husky Stadium in Seattle, Washington. 

A typical runner needs between 15 ounces (for a smaller runner) and 30 ounces (for a larger runner) of fluid every hour, Dr. Harrast says. To get a more precise measurement of how much water you need, weigh yourself before and after an hour-long training run to see how much fluid you lost.

As far as food, you’ll want to start taking in calories in the form of simple carbohydrates after an hour to an hour-and-a-half of running, Statuta says. Simple carbs are sugars that break down quickly in the body, rapidly providing you with fuel in the form of glucose, per the Cleveland Clinic. Energy bars, energy gels, crackers, and pretzels all qualify as simple carbs. 

RELATED: What to Eat Before and After Your Workout

Mile 10
The miles are flying by until your stomach starts to cramp — ouch!

Research suggests that between 30 and 50 percent of distance runners experience gastrointestinal (GI) distress.[1] There are two main theories for why that’s the case, Harrast says. The first is purely mechanical: Your gut jostles with every stride, irritating it until a bathroom break is required. The second is that blood gets diverted from our organs and sent to our muscles, interrupting the digestive process and leading to cramping, diarrhea, acid reflux, and/or nausea. 
Specific foods may cause digestive upset in runners, too, Statuta says, so experiment during training to find what bothers your stomach the least. Still, there’s no promise your gut will behave. GI distress can happen at any time on a course, Statuta says, including before you cross the starting line (due to race-day jitters). 

An hour into the race is a good time to introduce a sports drink. Sports drinks contain water, vitamins, and electrolytes, which are minerals that maintain fluid balance and spark muscle contractions in your body. According to the Cleveland Clinic, you lose electrolytes — including sodium, calcium, and potassium — when you sweat.[2] Sports drinks may support athletic performance by replacing lost fluids and electrolytes. 
Statuta and Harrast recommend sipping a sports drink and water throughout the race. Alternate between the two to avoid hyponatremia, a potentially serious condition in which you become overhydrated and your sodium drops below healthy levels. According to a review, 7 to 15 percent of marathon runners develop hyponatremia, which carries symptoms like nausea, vomiting, confusion, muscle cramps, seizures, and even coma.[3]
RELATED: 6 Quick Tips for Running Your Best Marathon

Miles 12 to 15
As the miles increase, waste products (byproducts of the body’s energy-making processes, including lactic acid) start to build in your muscles, and the pounding of feet on pavement (or trail) starts to induce fatigue and soreness, according to Statuta.

You don’t use your muscles as effectively when fatigue sets in, Harrast says, which can lead to strains, injuries, and cramping. The best solution is prevention — be well-trained for race day. Harrast also recommends changing your stride for short bursts every mile or so to give your body a new stimulus. Take bigger steps for a few seconds, and then take smaller steps. 

Hydration and fueling also affect the comfort of your muscles, so stick to your food and water plan.

Miles 18 to 20
Things are going great, and then suddenly your legs turn to lead. Each step feels like running uphill. You’ve hit “the wall,” as runners like to call it. Some argue that the wall occurs when you deplete your glycogen stores (glycogen is the stored form of carbohydrates), while others, like Statuta, suggest that your body just moves slower as you fatigue. 

There’s also a mental factor to consider. This is likely the farthest distance you went in training, you’ve still got miles to go, and your brain is screaming, “Why are we still doing this?!” 

If you do hit the wall, one small study in competitive athletes suggests that taking in more carbohydrates may help.[4] Also, try distraction or positive self-talk to power through. And by mile 23 or 24, the roar of the crowd should help put some pep back in your step.
The Finish Line
The marathon is over, but your heart, lungs, and blood vessels are still amped up. It’s imperative to keep walking even after you cross the finish line — otherwise, blood can start to pool in your lower extremities. “The muscle tone relaxes, the vessels relax, so blood gets pulled down and away from the brain,” Statuta says. When blood is redirected from your brain to your legs, you can become lightheaded and even collapse. To prevent this, the American Heart Association (AHA) recommends walking at a slower pace for at least 5 to 10 minutes.[5]
Meanwhile, the kidneys are working hard to filter waste from your blood. In fact, research suggests that marathoners suffer mild kidney damage on race day.[6] That’s because the body diverts blood from your organs to keep you moving forward, explains Harrast. Dehydration can restrict blood flow through the kidneys, too. The good news is your kidneys repair this damage on their own, so you don’t have to do anything special. 
However, if your urine turns brown or you’re having severe muscle aches or cramps, you may have rhabdomyolysis, a potentially life-threatening medical condition. Rhabdomyolysis causes your muscles to break down, leading toxic components of your muscle fibers to leak into your circulation system and kidneys. This can cause kidney damage, per the Cleveland Clinic.[7] Seek immediate medical care if you notice symptoms.
Your legs aren’t the only thing exhausted — your immune system can weaken during the three to 72 hours after a marathon, Harrast notes. But your immune system being down isn’t enough to give you a cold — you still need to be exposed. All those people hugging you and high-fiving you at the end? They could be passing on a virus. Healthy practices like getting enough sleep, eating well, avoiding crowds, and practicing good hand-washing habits are key to protecting yourself from post-race sniffles, Harrast says.