i've been riding mountain bikes since the mid-80s. they are what took the place of dirt bikes when i first moved to the states from new zealand, and then they became a source of income in one way or another, and have remained so since then. i've mellowed out a lot in the last decade, and gone from pretty fit race shape to regular rider with about 10 more pounds on my middle than i want. but i can still rip out a 30 or 40 mile dirt mtb ride without feeling too bad.
the weird thing for me is that there is some crossover benefit in terms of riding dirt bikes, but all the mountain bike miles don't do a damn thing for me when i get back on the moto after some time off. i went out friday for five hours on my brand new leftover 2010 300, and it kicked my ass seven ways into next week. the mid-deep isometric squat that is part and parcel of dirt biking hurts my legs in ways that pedaling will never help, and no amount of singletrack on a bicycle will cure the forearm pump at the beginning of the ride and the tired shoulders at the end.
only cure for me is to ride more dirt bikes! could be worse fates.