The way points of this bicycle route:
Turn left onto the trail to Crystal City. The trail leads to a tunnel under the railroad tracks.
After exiting the tunnel, turn right on Crystal Drive
Turn left on 12th St S
Turn right on Old Jefferson Davis Highway
Turn left on Boundary Channel Drive.
You can go right here and across the pedestrian bridge to Columbia Island, then through the parking lot and cross the GW Parkway at grade, but I wouldn't.
Turn left at the end of the parking lot
Turn right up the hill on the entrance ramp to 27
Get on the sidewalk which is a feeder trail to the MVT.
Rejoin the Mt Vernon Trail