I too am arriving from Europe in early April to IAD and plan to just catch the 5A Metrobus to Rosslyn Metro. It is an Express so fare is $3, then additional I assume onto the Metro. (Or one can stay on to L'Enfant Plaza Metro on almost all the Metro lines.) If you have a lot of luggage, that will be a problem: I just have a small roll-on and shoulder bag so have always been able to manage on public transit buses and subways.

If you can take the 5A, it departs at 11:35a, 12:35a, 1:35p and 2:35p, then 3:40p and every hour thereafter until 11:40p from Curb 2E.