The Potomac Heritage Trail is a favorite go-to run, offering nearly 20 miles of uninterrupted singletrack within the Beltway. The trail is an out-and-back, starting at Teddy Roosevelt Island and turning around at the Beltway at Cabin John Bridge. On any given weekend you are almost guaranteed to run into a club member, especially Ed Demoney, on the Potomac Heritage Trail.
The entire run generally follows the blue-blazed Potomac Heritage Trail. 7.1 miles from Roosevelt Island, one can make a left onto the side trail that climbs into Turkey Run Park, which has restrooms and water (in season). Continue through Turkey Run and rejoin the Potomac Heritage Trail after a mile. This is the route shown here.
For variety on the way back, or if you want to end your run with a tempo run, cross over Chain Bridge and return to Roosevelt Island on the C&O Canal.