RamJam, you should try either Boyce Park in Monroeville/Plum or Pleasant Valley Park in Murrysville.
PV is very suited to a beginner to intermediate rider and the trails are VERY well maintained with minimal technical work and obstacles. The park is on Pleasant Valley Road (off Business Route 22). Search the web and you can find trail maps at pvpark (dot) org
Boyce has something for everyone and you can spend all day there and not ride all the trail. It's easy to get turned around at times and you can find yourself in a hairy spot now and then if you don't know where you're going. That said, park at the soccer fields and go explore. You won't be disappointed.