They're called ultra-marathoners, and in their world, the toughest race of all is the Badwater Ultramarathon through a scorching desert and halfway up a mountain, all without stopping.
Last summer, 72 runners from 11 countries qualified for the race. 60 Minutes focused on the two favorites, who have had a most unexpected rivalry.
Dean Karnazes, 41, at 5'9", and 155 pounds, is a real contender. His big competition is Pam Reed, 44, who is just 5'3", and 100 pounds.
Karnazes and Reed start the race at 10 in the morning at Badwater. It'll be a 135-mile course that winds through Death Valley , a stretch of the Mojave Desert that is considered the hottest place on the planet. Temperatures can reach over 130 degrees.
The asphalt gets so hot here – up to 200 degrees - that runners stick to the white line to keep their shoes from melting. How do runners keep their feet from blistering?
"You don't," says Reed. "Part of long-distance running for me is just, it's pain management. Just, you know, you're going to have it."
"You'll get a little calf pain, and then your knee will bug you a little," adds Reed. "And then your neck will hurt, and you'll get a headache, or that'll go away. And it just, it moves around."
So you just move through it? "Yup," says Reed.
The race goes on for 30 non-stop hours. The Badwater race starts below sea level and goes up 8,400 feet to the finish line halfway up Mount Whitney, the tallest peak in the lower 48.
"Badwater has pounded me into submission," says Karnazes. "I mean, it just dropped me to the pavement at mile 70 the first year I tried it."
Why does he feel the need to put himself in conditions that a normal person wouldn't even survive? "I think it's exploration," says Karnazes. "I mean, I have a-- just a desire to see how far the human body can be pushed."
So how do you train for a race like this?
Reed, from Tucson, Ariz., has an unorthodox approach. With no coach, no nutritionist and no training schedule, she simply runs as much as she can – up to five times a day. For a mother of three, that means in the middle of the night, in between errands, or during her son's soccer practice.
In San Francisco, Karnazes trains just as hard. On weekdays, he runs up to 30 miles before his wife, a dentist, and their two kids get up and around 100 miles on weekends.
"Most people, when they hear that I've run 100 miles, the most frequent reaction I get is, 'I didn't know human beings could do that,'" says Karnazes.
It is hard to believe, as is just about everything these ultra-marathoners do. When Karnazes trains, he's out the door before dawn, running through the city streets, over the Golden Gate Bridge and into the hills.
It's believed that Karnazes has run farther than any other living person -- 262 miles in 76 hours. That's three days without sleep. But this is actually his hobby. He's president of a health food company, and sometimes, he even does his business in the middle of a run.
At the 20-mile mark, Karnazes stopped at a 7-Eleven and he wasn't the least bit out of breath. "I usually can go maybe 10 hours without feeling too much," says Karnazes. "And then, after that, it goes downhill."
At this stop, Karnazes only bought coffee. But on runs where he can burn up to 30,000 calories, he's been known to down an entire cheesecake. And get this: He's even ordered in.
"I run with a credit card and a cell phone, so when there is not a 7-Eleven around, like some of the country roads out there, I can get him to deliver a pizza to me. And I kind of give them a coordinate, a corner," says Karnazes. "It happens quite often, actually."
At Badwater, Karnazes covers up in the scorching noonday sun. He and all the runners have their crews follow along in air-conditioned vans, handing out everything from baby food to Red Bull.
By afternoon, everyone is suffering. Two runners have dropped out, and ambulances patrol the course, looking for runners in trouble.
Karnazes' blisters got so bad in one race that he had to stop for some first aid. "He [the doctor] lanced my blisters … and he stuck in a tube of Krazy Glue into the lance, and just pushed it down, and then put a piece of duct tape over it. And said, 'OK, get up soldier. Seventy more miles to go.'"