It's difficult to get the long endurance training in on a MTB..unless of course you ride it on the road.
Almost all of the top pro MTBers train a lot on the road.
Having said that, there's a lot of technique, skill, and pure low speed grunt that's necessary to ride well off-road. The off-road endurance races are crazy; no way in hell I'd survive 12 or 24 hours on single track.