well said JM, Fed might be one of the most hard working players out there, this is very rare among "talented" players, just look at Gasquet (the party boy) or Nalbandian (more burgers please) and the like, for example after MC he had 4 hours of extensive training and practicing and physical conditioning EVERY DAY until Rome, not to mention the unbelievable work and preparations he puts in the off-season.
there is a reason why Federer is able to outlast his opponents and never be tired, and no it isn't genes.
He needed that training though. Check page 43 of 'Male Player Bulges' in Non-Tennis for evidence. He had a serious spare tire going into clay season and wasn't as fit as he used to be in spring 2007 and throughout 08. It made sense that his back was bothering him.
Right now he looks the fittest he has in years. 2006 at least.