67y/o here, who did the same thing about two years ago. Used to put in about 120 road miles per week. Now, I still try to ride daily, but if I cover 6-10 miles per session it's a big day!
I've found that the two disciplines require very different types of fitness. I could ride the roads at 14-18mph for a couple of hours and never run out of wind, but 1/2 hour of jumping logs, crossing roots & riding skinnies and I'm sweating & sucking air. MTBing is a more whole body workout, and for me, much more aerobic.
I can't go to the park every day to ride, so I started in my driveway, too. There's more than just windsprints to be had there. There's wheelies and manuals and fakies, there's precision riding drills (following a straight line or turns with exact wheel placement). If you can't hold a trackstand for 15 seconds now, practice in your driveway...you'll find it useful in the woods.
I have a tiny back yard, but I ride there, too. It has a whole course of home built obstacles. Pictures are on a different device, but I'll try to post again to show what I did later. That REALLY helped my time in the woods as it's given lots of practice on approaches and follow throughs to obstacles (really important to success). The back yard features include split firewood lines and curves to simulate roots, treated lumber skinnies, ramp jumps, platform jumps and log jumps. All obstacles are not terrifyingly large to avoid major injury in event of failure, but demand proper technique for success. Pretty fun and right here at home!
Good luck, you'll find plenty to work on!