As you cross the Anacostia, you have a pretty decent hill in front of you. Then a longer downhill. After that, it is pretty good cruising. Lights are more spread out and you have a shoulder. The hill coming back towards Capitol Hill in the afternoon will be worse.
Have you driven your route by car? I know how much it sucks to postpone a ride after anticipating it, but I would highly recommend driving it first. You are going to have to take the lane a few places and it's good to see the conditions of the roads and turn lanes.
Have you checked the mileage? Vienna on the WOD is at mile 12; 19 miles from Capitol Hill. I've only driven a car on the stretch of Penn towards Maryland, so I can't estimate the miles, but it seems like it would be longer than 6-8 miles. I could easily be wrong though.