While I was back in Virginia, I did a run on Fairfax County's Cross-County Trail [site has maps that might be interesting if you've ever lived in the area]. When I discovered it was only 10 miles from my old house along the trail, which follows the Difficult Run, to the Potomac River in Great Falls National Park, I had no choice in the matter and ran there and back. Biking and running, it's amazing how tiny all the hills that I thought were gigantic as a kid really are.
The CCT is a very cool trail. Despite how developed Fairfax County is, apparently the flood planes have always been public property, so there's lots of open spaces along the streams. It's very well marked, and at least on the portion I ran, very little is paved, and it was also fun to see the ultimate destination of the creek I used to play in all the time.
Another cool trail in developed areas I've just learned about is the East Coast Greenway.