On of my favorite places to run is the Montour Run Trail, near Pittsburgh, PA. Whenever you can catch it on a beautiful fall afternoon, like the one we had today, it can't be beat for a place to click off some miles and unwind after work. The trail section that I use whenever I'm in town follows the run (a creek to us Southerners) past wooded hills, over a rail bridge and through a train tunnel. The surface is near perfect, the grade near flat, and you almost always cross paths with fellow runners, cyclists, dog-walkers cross-country teams or power walkers. There are parking spots interspersed along the trail for easy access. I always hope for decent weather just so I can run here.
Update: Added photo