Around here most trails tend to require moments of exertion followd by periods of rest but the roads are all exertion all the time. So the environments and the demands on your body are different here. Mountain biking seems to favor anaerobic exertion and road biking aerobic.
Also, don't neglect the impact of slowing down for terrain and curves. It takes a lot of energy to re-gain that speed and you lose a lot of time.