Average speed (and, therefore total distance) on a mountain bike is VERY dependant on the technical difficulty of the terrain. I’ve come up with some factors in order to reconcile road distances with off-road distances. Without further ado:

-Very technical off-road riding = 4:1
-Technical off-road riding = 3:1
-Double Track/Dirt roads = 2:1

For example, in very technical off-road riding, 1 mile = 4 miles of road riding. While in technical off-road riding, 1 mile equates to about 3 miles of road riding, etc. The only real issue is identifying the technical merit of a given off-road ride. When I state that an off-road ride is very technical it must have a lot of climbing, a variety of obstacles and some tight single-track both up and down hill.

Anywho, that’s just my opinion … I could be wrong.