No one's mentioned the Justus Trail along the Allegeny River in W. PA. It's a paved rail trail that goes from Foxburgh to Oil City along the river with very little development. Just North of Oil City is the Oil Creek State Park Trail. The Justus Trail is flat, has two tunnels and there are several other trails that have access to the main trail. The Sandy Creek Rail Trail follows a winding valley and has some spectacular views. Food and lodging is available at either end of the Justus Trail. Our family has a cottage on the river near Emlenton and I will attest the area is beautiful year round. I especially like road biking in the area as the hills are challenging, roads are good and traffic is light. I do ride on the trail on family rides or when I do Z3 training intervals. Zipping through a dark tunnel at 20 mph is always fun, provided my light is working.
http://www.avta-trails.org/index.html

I will be riding the area around PA's Pine Creek Trail in early April. I am attending "hill climbing camp". We are staying at the Petty Coat Junction Campground on Ceder Creek near Watsontown. The campground borders the Pine Creek Trail. Three days of hill rides, hill repeats and uphill TT's, yea!!! I don't know if we will spend anytime on the trail since we will be on roadbikes and the trail is packed stone.
http://www.4thehealthofit.net/spring_cycling_camp.html