Some hope to break a record, but most just want to say they finished all 26.2 miles. Last year, two people died trying.
Seven more runners have died this fall in full or half marathons. Most of them were young and healthy, like 23-year-old Peter Curtin, who ran the Baltimore Marathon this month.
That sent shock waves through the running world and led some to ask if marathons were good for a person's health.
Statistically, the risk of death is less than 1 percent for every 100,000 runners, reports CBS News correspondent Kimberly Dozier.
They take no chances at the annual Marine Corps Marathon, held last Sunday in Washington, D.C., where thousands of spotters watch every runner, every step.
"Anyone who's staggering, that's usually the first thing you'll see, a little wobbly when they're running or walking," said Capt. Bruce Adams, a doctor with the Navy.
Doctors treat a few hundred runners every year, most for aches and pains. Some cases are much worse. These cooling stations are for heat stroke victims, whose temperatures soar to as high as 108 degrees. But what often kills is what you don't see: hidden heart disease, brought on by the stress of the race - and running it too fast.
A Canadian study showed marathoners' blood post-race was like that of a mild heart attack victim. Yet, you don't have to have a medical check up to sign up.
"To go to the depths that you'd have to, to screen everybody to be able to pick out those things, is not very practical and easy to do," Adams said.
One marathon-running doctor's advice?
"I don't recommend that people run marathons," said Dr. Paul Thompson, the director of cardiology at Hartford Hospital in Connecticut. "I want all my patients to walk a minimum of 30 minutes every single day."
If you have to run, he says to put in enough training time to make it across the line safely.