Start in the southern end of old town Alexandria, down near the Wilson Bridge. Run north along the waterfront and Union Street. At Oronoco Bay park, continue north along the river and pathway which will connect to the GW Parkway trail just south of the powerplant. Continue to T.R. Island after approx. 8.5 miles, take the bridge to the island and do the 1.5 mile loop (or twice to make this a 20 miler run), then leave the island and retrace your route back south.
I love this run: it's generally flat and fast and so varied. The run progresses from its busy, urban start to a calmer, if bike-busy, trail atmosphere. After the rush of running under the airplanes taking off or landing at Reagan, the crowds die off a bit as you head northwest along the Potomac. T.R. island's gentle trails and isolation allow you to really enjoy the nature around you and you mentally feel a world away from the metro area. A great way to spend a morning.