Your fitness is probably not as bad as you think - technique is HUGE in Mountain Biking. I'll give an example, I took a tri-athlete friend Mountain Biking last year...he does 4 ironmans a year and trains like crazy. He is in WAY better shape then me. But he was a novice Mountain Biker and was working 10x harder than I was on trails that I thought were low-intermediate in difficulty. On the road he could kick my butt, offroad, I was king...