There is a three hour ride on the training plan for Sunday, so a few of us are planning to do one right after the transition clinic at Cathlolic U. The tentative plan is to make our way from Catholic over to the Mt. Vernon trail, and then probably ride most or all of it (I think it's 36 miles round trip).
Info on the Mt. Vernon Trail (I've never been there):
We were thinking we'd just ride through town over there, but there would also be advantages to just driving over there (like avoiding tourists driving fully insured rental cars).
It will probably be a busy Sunday out there, but we need to get our three hours in! :-)
Another option on getting to the trail: hop onto the Metro from Catholic--Rosslyn is right at the head of the trail.
The Mt. Vernon Trail is 18.5 miles long. It is good for a slowish ride. I can't manage more than about 12 mph on it due to the turns and the people on it. But it is fun and scenic, and you can stop off at Mt. Vernon itself for coffee if you want.
Another possible option, pretty close to CU, are the very scenic and little known Upper Northwest and Upper Northeast trails, which connect all the way to Rock Creek in the west and the waterfront to the east: