I haven't mtn biked consistently for a really long time, but I always felt it required more bursts of effort combined with consistent effort on climbs vs a long endurance ride on a road bike. (More grinding it out, so to speak.) 
Both are hard in their own right and there's a reason gravel bikes are so popular right now. 

Rolling resistance is definitely easier on a road bike. One misconception is that the skinnier the tire and pumped to the max psi is more effective/efficient. But there are some good studies that show wider road bike tires and exact psi dependent on rider weight and type of tire, offer less rolling resistance. (Still obviously thinner and smoother than mtn)

6.5 total hours (including rests) is pretty aggressive. 6.5 hours moving time (not including rests) is definitely doable for someone like you. You'd probably be closer to 5.5-6 hours moving time.