In "Pushing the Limits," a new "CBS This Morning" series, we're profiling seemingly ordinary people doing remarkable things.

If you think running a marathon is a feat in itself, imagine doing an ultra-marathon up a mountain.

At "The Rut Mountain Race" in Big Sky, Montana, nearly 400 athletes from around the world gathered to compete across 31 miles. They cover more than 10,000 feet of elevation. The sport is known as "skyrunning." In this edition of "Pushing the Limits," we take a look at the world's elite athletes who are getting their high-altitude adrenaline rush.

Luke Nelson

Luke Nelson CBS News

"I run in the mountains because it gives me a freedom I can't feel anywhere else," the professional mountain runner said.

Nelson said there is a lot more elevation gain and loss than usual marathons and the races are more technical.

"It's pretty gnarly and pretty brutal, but it's a lot of fun... It's a pretty phenomenal sport and it's exciting," he said.

Emelie Forsberg

"Skyrunning is not hard for me because this is what I do," Forsberg said. "I have been running outside in nature and mountains for my whole life, so for me, it's not hard."

The Swedish runner is both a European and World Champion in long-distance skyrunning.

Max King

"I'm definitely pushing to my limits every one of these races that I do," King said.

Ian Sharman

"To hike the 50K, the 31 miles here would probably take the average person more than a day...and these people are doing it in about five and a half hours," said Sherman, who is the director of U.S. Skyrunning.

Mike Foote

"We have some of the best mountain runners in the world today competing... and it's been really inspiring to watch them compete," said Foote, ultra-marathoner and race director of the Rut Mountain races.

Watch the video to see how the skyrunners endure the race.