wait a second....something doesn't compute. if you can twice ride a sustained 18 mph on a mountain bike for an hour each time and at neither point raise your HR to above 65%, then i would say you are plenty fit. 65% MHR is the zone at which you build a base, and is far below your LT.
I peak it above 65 occasionally - one of the major hills, but most of the time it stays below, but im still feel like i ran a marathon when i get back lol.