Welcome to the road.
I was all mtb all the time from the mid 80s through the mid 90s.
Then, I got tired of all the crowds of poseurs showing up on my trails along the wissahickon in philly, and started riding my mtb on slicks on the road with my roadie and cross friends until I finally decided to go all road all the time and bought a lemond road bike.
On the road, the workout is definitely not as all-body as technical off-road riding, but the thrill of speeding along, hammering for hours on end, more than makes up for it.