Lots of sports start off being mostly about the physical challenge for the beginner with the mental side becoming more important as you become more proficient. When it comes to riding a mountain bike downhill though, it seems to me as though the opposite is true. While it may look like a physical activity the challenge for the beginner is almost entirely mental. It's only at higher levels where physical condition starts to make a meaningful difference.
After more years than I care to mention riding road bikes, I've spent the last couple of years trying to get to grips with a mountain bike. I've been on a few skills courses (group and private), read loads of stuff and spent a fair few hours trying (with limited success) to put it into action. I've come to the conclusion that there isn't actually much to riding a bike down a hill (at least at my level). If you point it roughly where you want to go, stay off the brakes, stay loose, let the bike move about and trust that it will get down then it usually does. That trust is the tricky bit, but nine times out of ten when I crash it's because I've lost confidence that the bike will get through on its own and tried to intervene. Unfortunately that one time in ten when you crash because your confidence was misplaced tends to be the one that requires a trip to A&E to put you back together, which makes trusting the bike all the harder, but I still think that's the key.