One of my favorites is the Black Forest Trail in the Tiadaghton State Forest. 42 miles, lots of steep ups and downs, great views. A great challenge if you're in for a challenge. There's a decent hotel a couple of miles from where the trail crosses a road near, I think, mile 22. It's where I often start (instead of starting at marked mile 1). Saw four bears on one trip. http://www.pahikes.com/trails/black-forest-trail
Allegheny Front Trail is also a nice trail. Just did this in, I think, October or thereabouts. Not as challenging as Black Forest, but a nice, enjoyable trail nonetheless. Throws a lot of looks at you. http://alleghenyfronttrail.info/
Both of these are about 4-4.5 hours from DC.