So you think you’re fit? If you’re comparing yourself to endurance athlete Dean Karnazes, you may want to think again. He makes even dedicated performers look like couch potatoes.

Time named the LA-raised go-getter of Greek extraction one of the “Top 100 Most Influential People in the World,” Men’s Fitness hailed Karnazes as one of the fittest men on the planet and Wired described him as the “perfect human”.

No athlete comes closer to being superhuman. Besides having completed 50 marathons in 50 days, Karnazes logs 200 miles for a laugh and races in 120-degree heat.

Not bad for a former marketing executive whose idea of competition was once the rat race. A graduate of San Francisco’s McLaren School of Business and Management, he has worked for several Fortune 500 companies.

Karnazes started running when he was very young. Back in school, he met track coach Jack McTavish, who became his mentor and introduced him to the Spartan joys of long-distance running.

McTavish’s outlook was simple: “Go out hard and finish harder.”

It worked. Karnazes duly won the one-mile California State Long-Distance Championship. When Karnazes said to Coach McTavish that the race felt “pretty good,” McTavish replied, “If it felt good, you didn’t push hard enough. It’s supposed to hurt like hell.”

Karnazes’ passion for endurance running re-ignited on his boozy 30th birthday, when, after midnight, he ran out of the San Francisco bar where he was celebrating and continued all the way to Daly City. He covered the 30 miles in his underwear and a pair of battered lawn-mowing shoes.

En route, snapping out of his tequila trance, he had a revelation. He realized that he contained vast, untapped reservoirs of strength. He decided to embark on a new course, or end his party animal phase and revive an old ambition.

Karnazes, however, doesn’t always play by the rules. He subjects himself to sleep deprivation and has pushed his body to the brink of death. During races he scarfs extra-large Hawaiian pizzas chased with cheesecake, while, when he’s not racing, he eats like a monk or supermodel, picking at grilled salmon, dark rye bread and apples. His “slow-carb” diet keeps him on an even energized keel.

And just to top it all off, Karnazes is also a talented, highly quotable writer.

Here are three motivating statements that the star of sport and business made in his book, Ultramarathon Man: Confessions of an All-Night Runner:

1. “I wasn’t born with any innate talent. I’ve never been naturally gifted at anything. I always had to work at it. The only way I knew how to succeed was to try harder than anyone else. Dogged persistence is what got me through life. But here was something I was half-decent at. Being able to run great distances was the one thing I could offer the world. Others might be faster, but I could go longer. My strongest quality is that I never give up.”

2. “Most people never get there. They’re afraid or unwilling to demand enough of themselves and take the easy road, the path of least resistance. But struggling and suffering, as I now saw it, were the essence of a life worth living. If you’re not pushing yourself beyond the comfort zone, if you’re not constantly demanding more from yourself, expanding and learning as you go, you’re choosing a numb existence. You’re denying yourself an extraordinary trip.”

3. “As long as my heart’s still in it, I’ll keep going. If the passion’s there, why stop? There’ll likely be a point of diminishing returns, a point where my strength will begin to wane. Until then, I’ll just keep plodding onward, putting one foot in front of the other to the best of my ability. Smiling the entire time.”