Black Forest Trail is one of my favorites, but there is a significant water crossing unless you take a detour. Also take a look at the Quehanna Trail (while it is a 75-mile loop, there are tons of side and crisscrossing trails that would allow you to shorten it up to whatever you wanted it to be), and the Allegheny Front Trail, app. 40 mile loop. I did the Allegheny recently and had a great time, though it was a bit muddy.
Also, since you're in Philly, you might want to look at Delaware Water Gap. It's app. 40 miles from High Point State Park, NJ to Delaware Water Gap. It's a very nice trail, not too difficult, with some really nice views.