In the world of long-distance running there are the marathoners, ultra-marathoners and the desert runners who trek for days through drought, wind, heat and cold with supplies strapped to their backs.

Fitness experts say for athletes grappling with these extremes, success is more a matter of can-do spirit than physical prowess.

“The people who finish are not the most physically fit but the ones that are mentally strong, those who don’t entertain the possibility of not finishing,” said ultra-marathoner Samantha Gash.

Gash, 29, was the first woman to complete the Four Deserts Grand Slam, an ultra-marathon series where runners slog across four 150-mile courses in deserts in Chile, China, Egypt and Antarctica.

Organizers of the series, which was founded in 2002, said so far 28 people have completed the Grand Slam.

A self-described “sucker for new experiences,” Gash, who hails from Melbourne, Australia, is featured in the recent documentary “Desert Runners,” which chronicles the Four Deserts Grand Slam.

What extreme athletes share, she said, is ambition and a willingness to step outside of their comfort zone.

Despite holding down a full-time communications job, Gash is currently training for a 32-day race in September along South Africa’s Freedom Trail, to promote awareness of the lack of female hygiene products available to young girls in that part of the world.

"I'll wake up at 3:00 a.m., run 24 kilometers (15 miles), do an hour (of) strength training, run another 15 kilometers (9 miles) at night,” said Gash, who also does yoga.

Truly addictive is how Connecticut-based running coach Tom Holland describes the extreme experience.

Holland is a veteran of several ultra-marathons, including the “Run to the Sun,” a 36-mile jaunt to the summit of the 10,023-foot summit of Haleakala on the island of Maui.

“Freud said humans seek pleasure and avoid pain. We seek pleasure thru pain,” said Holland, author of “The Marathon Method.”

Holland said he has seen elite runners put treadmills in the sauna to prepare their bodies to run in the heat.

But regardless of how bad you feel at the finish, he said, the confidence gained lasts long after the race and translates into all other aspects of your life.

“In a bizarre way you find out who you are,” he said.

Gregory Chertok, a New Jersey-based trainer and psychology consultant for the American College of Sports Medicine, warned the going to extremes can be dangerous.

“Several ex-runners have commented on mourning the loss of running like a family member,” he said. “There comes a point where you must ask ‘who is really in control, the runner or the running?’”

Chertok noted a study published in the peer-review journal Mayo Clinic Proceedings found that the cardiovascular benefit of vigorous exercise built up over about one hour. Beyond that, further exertion produced diminishing, and possibly even adverse, effects in some people, he said.