Bob Brown (from Cornwall, England) won a 3,100 mile race across the United States, running the equivalent of nearly 2 marathons a day for 71 days in a row. Before that he ran the London Marathon in an old-fashioned 130lb diving suit (complete with lead boots) and has 'jogged' 135 miles through California's Death Valley in blazing heat in under 60 hours.
http://news.bbc.co.uk/1/hi/magazine/3590700.stm
When I finished Death Valley, I brought up my stomach lining. It was all acid. I burnt my tongue and lips. I lost three toenails, had heat exhaustion, nerve damage to both feet," Mr Scott recalls. "It was extremely brutal on the body."
Extreme athletes are constantly testing the limits of their bodies and can be hovering on the "knife-edge" that separates injury and illness from optimum health.
"It's over and above," Mr Dunbar says. "You often hear of athletes talking about being on the knife edge, of having a certain amount of fitness that takes them way beyond healthy, and then you can take it that little bit too far - for athletes, it's in terms of intensity - that can lead them to illness and injury.
http://news.bbc.co.uk/1/hi/magazine/3590700.stm