I’d have to agree with bike nerd about jumping into the mtb scene clipped in on a xc racer BUT maybe the trails you’ll be on are much more forgiving then what Im used to. I would say work on your slow technical skills like slow balance, hopping the bike over low obstacles, getting your weight back over the rear wheel on steep desents, if any. Ride different conditions and weather, be prepared to crash it will happen and better ya are at crashing less chance of injury. Tire pressure is a science in itself and can make a easy ride hard or a hard ride easy so read up and experiment. Learn something every time you ride as I never think I’m good at trails I just think I’m always improving my skills. Its a different discipline and much more rewarding. I gravel ride on occasion and the g bike gets me on the road once in a great while just for a short rip from the house is all. Dont be discouraged with fitness change either . I know a roadie will crush me on a 50 but i can still just up and do a 30 mile road or gravel ride however there is a good chance he or she would be discouraged on 1000ft elevation gain in a half mile over rocks and roots on a path no wider than a foot. Enjoy it, if you have trails and take a liking chances are you wont be riding road much more.