Fantastic thread. I was just hunting around the net for bike trails as that's what I want to do on the weekends when I have time, head out and explore the bike trails around my area of W. PA.
That said, I live near Emlenton and Foxburg PA which has the Allegheny River Rails to Trails. The 6 miles between the 2 towns is not paved, but is perfectly passable. It's just not a main part of the trail yet. If I don't have a lot of time, I stop in Foxburg and ride the 2.5 miles south to Parker and back as hard as I can. It is flat and has 1 bridge that crosses over the Clarion River. This section is so flat that I think it is downhill both ways.
My normal daily route is to go beyond Foxburg to Emlenton where the trail follows the river for I think 32 miles up to Franklin, PA. So far because of how slow my junk bike is, I typically ride 6 miles out where there is a tunnel which feels so nice when it's 90° out. I turn around and head back after going through the tunnel.
On weekends I can take a bit longer of a ride. My rear end can't handle much more than about an hour and a half, so I go about another 3 miles beyond the tunnel for the turn-around. That puts me past some nice camps and river homes along the river. That gets me about 18 miles in.
Thus far, 25 miles has been my longest ride and that took me out to the 2nd tunnel and back. The 2nd tunnel is curved and when you get into the middle, you can't see your hand if you're touching your nose. Lights are an absolute must have as you are in the middle and have no light from either end. It's really neat. I love the tunnels. This trail at least the short section I ride seems to be an imperceptible incline as you go upriver. On my cheap mountain bike I am generally cruising along about 14 mph on the way up and then cruising a steady 15-16 mph on the way back.
The other trail that I've ridden and that I like is Sandy Creek trail. It crosses the Allegheny River trail maybe 15 miles or so up river. I start on the other side of the Allegheny to cross on the Belmar bridge. Really cool crossing the Allegheny that high up. The trail is a pretty steady and easy 2% (I think) grade for 8 miles until it crosses a major road and I didn't realize it continued. Sandy Creek snakes around quite a bit so you have several small bridges that crosses the creek. I had a blast turning around then flying the 8 miles back with a steady 2% downhill. I started at the Belmar bridge which isn't the very beginning of the trail. I think there's another 3-5 miles worth of trail before you reach the river and the bridge if you start at the beginning. That was my first longer than 8-12 miles ride and I wish I started at the very beginning as I was itching to ride more.