WOD may be a mixed bag. I've got no problems with any of it in the day, but not so sure about night if I'm solo.

Where the trail runs thru neighborhoods in Arlington, Falls Church, Vienna, Reston, Herndon, Leesburg, and Purcellville it's partially lit and partially seen from streets. The stretches in between are pretty remote and dark. Might be nice under a full moon, but it's also lonely out there. Depends on your tolerance.

How about around the Mall and Hains Point for night riding. Well lit, patrolled and probably not much traffic.
Or some of the good biking roads in North Arlington and Falls Church, like Wilson, Military, Williamsburg Westmoreland, Great Falls etc. You could probably create a pretty good nighttime road ride.