kc0bbq

You can probably just use the Roam for your initial mountain biking with the stock tires - use it until you have some idea what, if anything, you'd want different. You (hopefully) aren't going to be doing anything to over the top until you get a little experience with it, anyway. On mountain bikes various features aren't necessarily there for the reasons a non-mountainbiker would think. Suspension is there to keep the wheels on the ground, for example, comfort comes from learning to correctly use your legs.

My first few times on decent single-track were tough. Not body wise, but brain wise. My heart rate was sky high because I had to overthink everything. Takes a bit of doing it before your brain can just settle in on the things that actually matter instead of worrying about every root or tree.

18km is a good distance for a first real ride, doesn't matter what pace. Like others have said, speed comes with practice. Though the perfect smoothness of fresh asphalt with low resistance tires on a road bike seems to magically give me an extra couple mph.

That's a good start. Don't really think about speed much, and don't try to increase distance too fast. The guideline is generally 10% per week in increase, but that doesn't help at the very beginning because you don't know where you should be, so like ill.clyde said, listen to your body.