I have been riding dirtbikes even longer than uni, and one of my favorite places to ride is the Allegheny National Forest system. So just recently, I figured I'd uni it! There's a few different trails that I'm familiar with, including Rocky Gap (two eleven mile loops), Marienville (17 mile loop and 40 mile loop), Timberline (goes on forever and ever), and Willow Creek (11 miles). From my experience, I recommend Rocky Gap or Willow Creek. Marienville and Timberline tend to be wide open and rather boring. There's also quite a few walking/mountain bike type trails, but I'm not familiar with these...yet.
I rode Willow Creek last year, and I don't remember it fondly, but I wasn't the best rider at the time and I hope to revisit it. I rode Rocky Gap twice just recently, and I enjoyed it immensly. It has two loops, but I would stick to the Chapman Dam side, as the other is nearly impassable. It is rocky and technical, but very fun. There's a few uphills I had to walk, but I did the trail in just under two hours. It's a lot of fun and I recommend it to any Muni rider looking for a challenge. It's about an hour and a half from me, so if anyone wants to meet up for a ride, I'm in