Dean Karnazes is a man who believes in testing his body to its limits. As an ultra-marathon runner, he regularly takes distances of 50 miles or more in his stride, running across some of the world's most inhospitable terrains.
He has run on every continent twice over and has just completed the 3,000 miles from Los Angeles to New York in 75 consecutive days.
He says his next challenge is to run a marathon in every country of the world in the space of a year.
Evan Davis ran a mile in his shoes to find out why he hit the road and what keeps him on it.