For decades, scientists have tried to understand what separates people capable of extraordinary endurance from everyone else. Is it genetics? VO₂ max? Pain tolerance? Mental toughness?

Dean Karnazes has spent more than 30 years giving them a remarkable case study.

He's run 50 marathons in 50 states in 50 consecutive days, crossed Death Valley in the middle of summer, and in 2005 set the record for the longest nonstop run — covering 350 miles in 80 hours and 44 minutes.

Even that wasn't enough.

"I actually made it 350 miles, which was somewhat of a letdown because I thought maybe I could run 500 miles," Karnazes told me. "That was kind of the ultimate goal." His world record still stands today, 21 years later.

After decades of ultrarunning, Karnazes says he's learned that doing hard things comes down to a handful of simple habits that anyone can practice.

Don't make winning the goal

Karnazes has finished on the podium plenty of times, but he says that's never been what pulled him to the starting line.

"I do it for the adventure," he said. "Both the inner and the outer adventure. I've never really said I want to win a race. I just thought, be the best Dean that Dean can be on race day."

He says approaching races that way has changed how he defines success. The podium, if it comes, is just the cherry on top.

That mindset, he says, has kept him motivated through hundreds of marathons and ultramarathons, long after chasing trophies stopped being interesting.

Redefine what success looks like

When Karnazes gives advice to new ultrarunners, he doesn't start with mileage or pace.

"Just set your goal on finishing," he said. "Don't say I want to do it in sub five hours or sub six hours. Just say, 'For me, success is reaching the finish line before the cutoff.'"

Karnazes says that approach has a way of changing how you see your own limits.

"You'll prove to yourself that you're better than you think you are. And you can go further than you think you could."

Keep moving, even if you have to slow down

The advice sounds simple, but following it is another story.

At some point during his 350-mile run, Karnazes stopped trusting what he was seeing.

After nearly three days without sleep, he said he started hallucinating. He felt as though he had left his own body and was watching himself from above.

"When you're in these altered states, it's as though you don't have conscious control of your mind," he said. "You're fighting your mind to stay present, but your mind gets away from you."

Despite that, he kept running.

I asked Karnazes for his advice to aspiring ultrarunners who worry they'll hit a point where the pain becomes too much.

"You walk if you have to," he said. "You slow down, you push through the pain, and you come out the other side, and you're rejuvenated."