Lots of good advice here! As a roadie who has dabbled in mountain biking a bit, I can say that you've already received some great advice. I will add one thing: it's always a good idea for a novice MTBer to ride with others (or one other) who has more experience. It really helps to watch what others do and follow their lines, and you'll get more advice from them as well. Using your roadie skills is not always what's needed; you'll need to be out of the saddle more and paying even more attention to what is in front of you (roots, rocks, etc.) than on a smooth road.

I am a pretty wimpy MTBer and will only ride easy single-track trails, but gravel roads and double-track trails can be lots of fun too.k Just don't forget that you can spin out and wipe out in loose sand and gravel even with the fatter tires of a MTB, so be careful out there and, like others have said, don't be afraid to walk, or even just put a foot down, assess the situation, and maybe start up again. It's also fine to "redo" a section you want to "get" if you are having a tough time with it; for example, a switchback. If you fail the first or first ten times, just go back and try it again. Practice, practice, practice. And don't forget that your low gears are your friends. There's a reason that MTBs have smaller rings up front and larger in the back! I know not using the granny gear is often a point of honor among roadies, but there's no reason to feel that way on a trail. Use granny whenever the trail goes up, is my philosophy. You'll lose momentum a lot faster than on the road, so you have to use your gears.

I would also recommend starting without clipless pedals. You'll have more confidence knowing you can put a foot down to stop a fall really fast. I didn't do this when I got my first MTB, but wish I had!