If you want fast, you're probably going to need to take a cab. Expect the one-way fare to be about $60. If you're traveling on a weekday, one other option is the 5A Metrobus, which leaves 7th & D Sts, SW at 4:50 am, or Rosslyn at 5:02 am, arriving at IAD at 5:42 am. The fare's $3.10. This won't work on the weekend because the first bus isn't until 40 minutes later. I suppose you could also take SuperShuttle, but they may want to pick you up as early as 4:00 am depending on how many stops they'll make after you. Metro to West Falls Church to the Washington Flyer Bus won't work as it wouldn't get you to WFC before 5:50 am, and the first Flyer Bus doesn't depart WFC for Dulles until 6:15 am, arriving 6:40 am.