I am by no means an expert because I didn't get back into MTB till this Christmas. But in that time I have been just getting used to my MTB. I agree with many that have stated that often you have to adjust your speed expectations, at least on the flats and climbing but more importantly for me anyway is getting the knowledge of your riding surface. If you are like me the roads you have been on with your road bike have almost been memorized bump for bump and pot hole for pot hole. My first few rides on my MTB it was learning the trail. Cresting the top of a climb and not knowing what is on the other side sometimes caused me to tense up and not put enough effort to get over that last few feet. Once or twice over the same trail and I roll over the top as if I had been doing it for years. In other words I have to get to the point where I feel as comfortable on my MTB as I am on my road bike. So far that hasn't happened but it is coming one step at a time.