Moutain biking usualy builds lot more muscle mass than it does increase one's cardiovascular fitness
Road biking is an intesnely aerobic activiy, it works your cardiovascular system like nothing short of good ol' running, but(like running) it often does very little to build strength
In fact, if a mountain biker(with strong legs) was to start to do exclusvely road riding at very high cadence he would actualy cause his body to make his legs get weaker(protien metabolization, and energy priotiziation...too complicated)
but, back to the real world, as it is you probably have much stronger legs than the roadies but less cardiovascular fitness. depending on what you want the fitness for i would reconomend different things. but if all you want is stronger legs, then stay low cadence and tell the roadies that you training for off road riding, not road racing. I'd make idfferent reconmendations if you want something different, like some cardiovascular fitness...but ive already typed too much
excuse the spelling