Welcome to the Old Guy Club! Welcome to the Dirt.
I ride a Roady AND Dirty, They are similar fitness below the waist but you will find your upperbody gets more work off road.
Train your Core, train your core train your core. Then train your core some more. After that its technique and just getting comfortable and confident on your rig. Your Mountain rig will fit you different than your road rig did, and you will control it differently. Typically you don’t move around as much on a road bike. On your Mtb you use your weight to keep the rubber side down. (Thus the full body workout)
Read through some of the foums and the sticky’s up top. There is tons of experience and VERY useful information on here.
Welcoem to Singletracks,