I personally believe that Ueli Steck is the fittest man on Earth.
He trains for speed ascents of the world's toughest mountaineering challenges. Imagine Kilian Jornet combined with one of the best rock climbers in the world.
Here's his week:
http://www.upskillclimbing.com/2011/06/ueli-steck-climbing-training-plan.html
He does 10 hours of running in 5 runs per week, all at high altitude in the Swiss Alps, running up to mountain passes an back.
He does 12 hours a week of rock climbing in 3 sessions.
His Wednesdays are 4 hours of rock climbing followed by a 2 hour mountain run.
He has most of the major speed records on the major Swiss Alps peaks including the Matterhorn. It takes most people 5 days to climb the north face of the Eiger - he did in under 3 hours.