For many, pushing the line between thrill and danger is exactly the point.

“The mind, the mental anguish, is actually even harder than the physical,” said Karl Hoagland, the publisher of Ultrarunning Magazine. “With every ounce of your energy and your being, you want it to be over and to stop in the worst, most primal way. We’re programmed for survival, for the mind to shut the body down when we’re on empty, so you have to overcome that. Ultra runners are trained to get their body closer to empty.”

The uninitiated may question the wisdom of disregarding mental cues that promote survival.

“Maybe it is really unhealthy, but for me, I’m curious,” Hoagland said. “I only live once, and I want to see what it’s like to go to these dark places, where we’re not intended to be. At his or her core, an ultra runner is meant to go beyond what is rational. It’s crazy, but that’s how we are.”

Roche said the events were a chance to explore limits on one’s own terms — like a dress rehearsal for real adversity.

“Life now is pretty sterile — a lot of the problems we face are manufactured,” Roche said. “When things go wrong in ultras, it’s very real and visceral, and you confront it. It’s life condensed down to a bite-size morsel that you can generally consume without consequence. If you don’t finish, it doesn’t actually matter.”

As the number of races grows in pace with demand, the reclusive sport is changing. Although races generally offer little in prizes, money and sponsorship have begun to trickle in, along with some of the governance concerns that have plagued mainstream road racing. Lance Armstrong, barred from competition in many sports, won a trail race last year, driving debate around future regulation of performance-enhancing drugs in the sport.

And professional runners are increasingly drawn to the longer distances. The 2008 American Olympic marathoner Magdalena Boulet won her 100-mile debut at the Western States Endurance Run last year and continues to dominate the field. The Olympic distance runner Kara Goucher, who grew up in a town near that of the champion ultra runner Scott Jurek in Minnesota, also sees a new athletic future in the sport. Although she is one of the most accomplished marathon runners in the United States, Goucher said she was daunted by the challenge of even finishing such a distance.