You're entitled to call Dean Karnazes crazy. After all, the 44-year-old ultrarunner from San Francisco has some iffy tics, like jogging 350 miles in one shot and, most recently, knocking off 50 marathons in 50 states in 50 days (and then announcing, on November 6, that he'll run home—from New York City). But hang out with the endurance dynamo and you realize he's not nuts; he's just happy. What's his formula for fulfillment? Outside managing editor Katie Arnold caught up with Karnazes for race number 22, and this as-told-to interview, in Albuquerque, New Mexico. She discovered—when three miles became 13 and next thing she knew she'd run her first marathon, without training—that Karnazes's energy can rub off on the rest of us.

I never really had a five-year plan. I just asked myself, "If I could script my perfect life, what would it look like?" It would be spending a lot of time with my family and my kids; doing what I love, which is exploring the limits of human endurance; being in the shape of my life; and dedicating myself wholeheartedly to my craft. I love nothing more than to run through the mountains for 100 miles. It's been the most rewarding experience of my life.

I ran a lot as a kid but quit in high school. I didn't run again until my 30th birthday, when I realized that I wasn't fulfilled and had to make a change. I walked right out of the bar and ran 30 miles through the night. That was 14 years ago, and the longest I've gone without running since is two or three days. For most of that time, I worked in the marketing department of a pharmaceutical company; in 2003, I cofounded an organic-snack-food company.

On a normal day, I get up at four in the morning and run 20 to 25 miles before making breakfast and taking the kids to school. My average night's sleep is about four hours.

On a normal day I get up at four in the morning and run 20 to 25 miles before breakfast. Then I go windsurf or bike.

Learning to sleep so little took me about a month. It was really, really tough. I used to set my alarm and force myself to wake up. But what I've found now is that those four hours of sleep are restorative, whereas the seven or eight I used to get was a lot of restless stirring around. I also started tuning in to my diet. I cut out most processed foods and refined sugars. It took me a month to wean myself from sugar. But once you get over the hump, you've got this constant high level of energy. And when you feel good, you're much more motivated. I cross-train three or four days a week, on top of running. I might go windsurfing or mountain-biking before picking up the kids. That's my life. It's frenetic, it's crazy, it's fast-paced. There's no balance.

I think Western culture has things backwards. We equate comfort with happiness, and now we're so comfortable we're miserable. There's no struggle in our life, no sense of adventure. I've found that I'm never more alive than when I'm pushing and I'm in pain and I'm struggling for high achievement. In that struggle, I think there's a magic.

Unless you're pushing yourself, you're not living to the fullest. You can't be afraid to fail, but unless you fail, you haven't pushed hard enough. If you look at successful people and happy people, they fail a lot, because they're constantly trying to go further and expand.

That's what ultrarunning can teach you: Nothing comes easy. Running 100 miles is a huge commitment. There's no way to fake your way through that kind of distance. If you take shortcuts, you pay the price. When you're out at mile 80 and you're ready to give up, you're thinking, Man, I compromised on my training. I should have done those extra ten miles. And I know I didn't. Or you're saying, I paid my dues, I laid out my training, I didn't compromise, I ran those extra ten miles: I know I can pull this off.

As with anything, you have to believe that you can do it. And I think that's a learned trait. If someone had said to me before I started running ultras, "You're going to go out and run 100 miles through the mountains," I would have said no human being can do that, let alone me. And then when you actually achieve that, it teaches you that you're better than you think you are and that you can do more than you think you can. All along, that's what I hoped the Endurance 50 would do—inspire others to push themselves and do their best. I believe that anyone can do exactly what I'm doing.