The Big Daddy MUP in Northern VA is the W&OD trail. You can catch it as far west as Purcellville, VA, and follow it right down into Arlington, where it extends on the 4-Mile Run Trail right down to the Potomac River near the airport. Then you can catch the Mount Vernon trail and either head toward DC proper and the monuments, or head out to Mount Vernon.

I ride these trails after work, or on my commute, so I'm sort of sick of them but they really are a fantastic amenity and well worth seeking out.