All that rubbing and pounding can cause tendons to rub over joints (called ITB or illiotibital band syndrome), inflamed muscles in the foot, foot stress fractures, skin chaffing on skin and nipples rubbed so raw from moving shirts that they bleed.
"You want to have as little on the body as possible," said Horne.
To help with chaffing, she uses a product called 'glide' on her skin. Luckily, Horne said, she doesn't have a problem with bleeding nipples.
"That makes me just shiver," said Horne. "It mainly happens to men because women have sports bras. A lot of the guys that I train with on my team wear Band-Aids to help."
Horne runs with the Team for Kids, which has raised millions of dollars to improve the lives of children in the United States, as well as in the Cape Town region of South Africa.
As for her black toe, Horne said she never lets it slow her down.
"It never really bothers me; it hurts in the beginning when my toenails turn black and blue," said Horne. "Another toenail grows underneath and the black and blue one falls off."
Black toe can also happen to mountaineers, hikers, and other athletes.
A great documentation of a black toe experience can be found on the Starling Fitness Blog.
In Williams' experience, most marathoners run through the worst of the problems.
"Some of them can be pretty hardcore, and nothing is going to stop them from race day, and they're just going to fight through the pain," he said.
Unfortunately for Horne, another problem faced by serious runners kept her from reaching her finishing time goal. Last year, Horne finished in 4 hours and 53 minutes.
"Runners' stomachs can be ... sensitive," said Horne.
The runner's runs is a common term for it, and many serious long-distance runners face the problem. Although doctors do not necessarily understand the cause, one hypothesis is the blood flows so much to the legs during a run that the intestines react and cannot function properly, according to the Mayo Foundation for Medical Education and Research (MFMER).