Loudoun county bus. I leave the house by 7, drive ~5 minutes to the lot, arrive in Pentagon City by 8:30. When I've had meetings in DC the commute's been about the same - the transfers are different. Leave work at 5, home by 7, sometimes a little earlier depending on which bus I catch in the afternoon.
This webpage pretty much outlines your options:
http://www.commuterpage.com/schedules/route.cfm?op=9
Riders are generally friendly, the buses are coach buses. The other riders got a little testy the other day when the (new) driver missed the exit off 267, though.
It stinks, but it stunk more when I used to do the 980 from West Falls Church to Herndon/Monroe park and ride then drove the rest of the way home (another option).
Recommend paying extra (my bus is $2 each way with SmartTrip, but I don't think you get a transfer rate when you switch to the train) to get public transportation to bring you as close to home as possible.
Your biggest problem is the buses only run during rush hour, so if you need to get home outside of those times you're kind of SOL. I know, Commuter Connections exists, but I never seem to have the kind of notice they require to get a "guaranteed ride home" - though I'm signed up just in case.