Black toenails are caused by pools of blood that accumulate under the nail from repeated bruising. "It never really bothers me; it hurts in the beginning when my toenails turn black and blue," 26-year-old Jessica Horne, who will run her second ING New York City Marathon on Sunday, told ABCNews.com. "Another toenail grows underneath and the black and blue one falls off."

There are also chafed nipples (more of a problem for bra-less men) and tummy troubles, according to ABCNews.com. "Runner's runs" may be caused by blood flow out of the intestines to the legs, the site reports, sending the digestive system awry. Marathoners should walk for 20 minutes after finishing to redirect blood to their internal organs, according to a handbook distributed to entrants.

On the more serious side, one-sixth of marathoners develop hyponatremia, potentially deadly dilution of the blood, from drinking too much water, a 2005 study in the New England Journal of Medicine showed. Runners shouldn't drink more than eight ounces of fluid every 15 to 20 minutes; adding salty beverages with carbs and electrolytes helps absorb the fluid, the handbook says.

Finally, athletes eager to indulge in a massage should wait at least two hours after finishing the race, the guide says. Massage doesn't help prevent soreness if it's given sooner.