The Power running site has a nice perspective on this:
Basicly sprint work, strength work and then trying to run your even AT race pace. Specificity is covered with the race pace efforts while power and foot turn over are improved with power and sprint work. I think this combo is where it's at for elite performance unless the event is very long in which case you want something like what Greg described, in which every non-essential scrap of tissue has been catabolized. Not a rosy picture but that may be what’s necessary for some events/activities.
I know Mark Twight commented that his long efforts (48-72hr climbs) improved Dramatically with the inclusion of CrossFit but he still needed the long efforts, almost more for mental toughness than physical performance.