I can definitely relate to this. I went through a 10 year phase of only riding MTB (mainly technical forest singletrack and the odd bike park) and thought I was reasonably fit. Then I got back into road riding and was surprised to find how much faster it made me around my local MTB trails. I knocked several minutes/hour off my best MTB lap times within a year of regular fast road riding. So obviously the road riding was making a big difference to my endurance power.