I'm training for my first marathon and scared of 'hitting the wall.' An expert told me how to avoid burnout.

• "Hitting the wall" refers to physically and mentally running out of fuel when running.
• An expert said it usually occurs toward the end of a marathon, after about four hours of running.
• She said properly fueling up, mentally resetting, and walking parts of the race can help.

On November 3, I'll join over 50,000 people in running the New York City marathon.

It will be my first marathon. While I'm dreaming about crossing the finish line, celebrating with friends, and scarfing down a full pizza on the couch the next day, one thought keeps disrupting my fantasies: "Hitting the wall" during the race.

I've never hit the wall before, but I've met enough marathon runners to know its lore. It's common for people to feel physically and mentally burned out toward the end of the marathon, at anywhere from 18 to 22 miles out of the 26.2.

With only a few miles left, some runners might feel like they simply can't go on. The most disciplined athletes I know have described this moment as dark and deeply existential, bringing them to tears. (Meanwhile, I can't make it past six miles without digestive issues, and I whimper at the mildest inconvenience. I don't stand a chance.)

"Most of the time when you hit the wall, you have one major problem: your gas tank is running on empty," Milica McDowell, a doctor of physical therapy who has run 13 Ironman triathlons in addition to marathons, told Business Insider.

McDowell shared a few things marathon runners can do before and during the race to hop over that wall.

Train in fueling up

Properly fueling up during a marathon takes some getting used to.

"When you're training for a marathon, you want to be training your eating and drinking as well as your running," McDowell said.

The two main things to focus on are hydration and eating enough of the right foods.

McDowell said that you should drink fluids such as water or Gatorade, about two to four ounces at a time, to avoid getting an upset stomach. Tolerating four ounces at a time takes time, so she recommends working up to it as you train.

A typical portable water bottle is 16 to 20 ounces, and cups at water stations are usually around four ounces, she said. Being mindful about how much and how often you hydrate is crucial before and on race day.

The second thing to focus on is consuming simple carbs and glucose at about 200 calories per hour while running. Most marathon runners do this with supplements, which come in various forms, from gels to chews. I've seen people in my running group experiment with different brands and flavors throughout training to find what works best for them, whether caffeinated gel or electrolyte beverage.

"The last thing you would possibly do is eat a meat stick or granola bar," she said. "Your body wants simple sugar — that is what your muscles run on, your blood runs on, your brain runs on."

Have a reset plan

McDowell said that because your brain also runs on glucose, you may feel a mental crash as well as a physical one.

Fueling up might help you feel better, but it won't necessarily lift your spirits, McDowell said.

Most new runners are running 10 to 12-minute miles, she said. That means they'll hit the 20-mile mark of the race around four hours in.

"Most people just aren't used to doing the same thing for that period of time," she said. On top of physical fatigue or cramping, they might feel restless, bored, or agitated, just wanting the race to be over =.

One quick trick is a physical reset. "Playing with your running form can be a really helpful distraction," McDowell said. That can include going at an even faster pace for a few minutes, taking shorter steps, or moving your arms differently — anything to change things up and get yourself out of a rut.

If that doesn't work, then the reset might have to be more mental, she said. The key is to know what will motivate you when you're down, whether it's a pump-up song, mantra, visual image, or prayer.

"All of those elements really align for the psychological and mental toughness aspect of doing something that long," she said.

Walk it off

My three goals for the marathon are to finish it, avoid injury, and run the whole time.

According to McDowell, I might want to reconsider that last one.

From her experience with coaching runners, she said a big part of overcoming the wall is having a positive internal dialogue. If you need to slow down to a walk but tell yourself that you failed, "all that does is just decrease your performance," she said.

She said most marathon runners — especially first-timers — walk at least part of the race.

"It's really common to walk through the aid station, it's really common to do a run-walk program," she said. "There's almost a permission slip to consider that having some walking during training, expecting some walking during the race, is very much normal and part of the process."

Yes, training and fueling up are necessary. But it's just as important to have compassion for yourself — especially in the tough moments.

"Those parts to me are just really sort of a special sauce in finding a way to A, enjoy the training and then B, enjoy the race day," she said.