The Greek-American ultramarathon legend and author of the global bestseller "Ultra Marathon Man" Dean Karnazes has pushed his body and mind to unimaginable limits.

He has run continuously for 563 kilometers, a distance equivalent to more than 13 marathons, without sleep, in three days. He has crossed Death Valley with the thermometer reading 49°C and has run a marathon at the South Pole in -40°C.

In one of his most recent feats, he ran 50 marathons in 50 states of the U.S. in 50 consecutive days. He completed his challenge with the New York City Marathon, finishing in exactly three hours!

Time magazine has included him in the list of the "100 Most Influential People in the World." This is his story.