The Washington and Old Dominion (W&OD) trail end in Purcellville is 7.6 miles from Stone Gables or a trailhead in Leesburg that is even closer. This trail is enjoyed by bikers and hikers alike.  The W&OD trail is 45 miles in length and begins in Arlington.

Check out Friends of the W&OD Trail site

The W&OD page on Wikipedia  (with mileage chart)

C&O Canal Bicycling Guide http://bikewashington.org/canal/canal_c.php