The video below serves as a bit of an introduction to the race. It is a trailer for a film called The Barkley Marathons: The Race That Easts Its Young. The documentary made the rounds at last year's film festivals and received some acclaim for its depiction of this epic event.
By the way, the 2015 edition of the Barkley Marathons took place a few weeks back, and not a single entrant was able to finish. The tradition continues at what could be the hardest race on the planet.