Good points, everyone.
I'd like to add that MX has an element of "fitness" that cycling (a la Lance) doesn't have: taking impact. Having a top-shelf VO2 max is of little help against jumps/whoops/ruts and impact with other riders, not to mention wrestling a XXX-lb bike. (What does Lance's bike weigh?) Taking impact is what makes sports like soccer, boxing, and horse racing (to reference a post above) so punishing, and it's one of the things (like talent and mental toughness) that repeated fitness studies can't really measure.