yeah road biking is good training for mtn biking, I'm now able to commute to class on my single speed (roadbike) and I find that to be really good training, road biking builds great endurance but doesn't always help you be able to put out a ton more power ie going up a really steep hill in the wrong gear, so it woudl be a good idea to do windsprints (NOTE: i know road bikers sprint, and can throw down alot of power, i also ride road, but if you don't train right you won't get that b/c of how much just spinning you do, your not always racing)

anyway if you ride ur mtn bike on the road the tires will wear faster which sucks if they are expensive.

if you can't afford a road bike or w/e but would like ot go faster, you could invest in another wheelset and some slick tires, that woudl make a big diff