Even is a trail is marked with difficulty, there may be riders graduating to riding more difficult trails-noone could know if you're one of those or not. I'm not a fast trail rider (not the youngest either!). I just try to get out of the way if a faster rider is coming up on me, riders coming downhill should yield to riders coming uphill. If you see someone apparently broken down, or perhaps in physical stress, offer help. Carry in-carry out (don't leave behind bar wrappers or anything that wasn't there when you rode the trail). Don't ride overly muddy trails--it not only tears up the trails, but in worst case scenario could possibly result in trail closure (some of my favorite trails when I lived near PA started posting whether the trail was open or closed to bikes, horses, hikers on a sign board at the parking lots). Try to ride the trails that suit your skill, and once you feel ready, step it up!