The DC Metro system has a bus to Dulles.
It's cheap, and there are, I think, only two other scheduled stops, but it takes an hour to get to Dulles from L'Enfant Station by bus.
You can check here for your best timetable. The Metro rail system map is here. The schedule of the 5A Metrobus to Dulles is here. They're all linked from the Washington Metropolitan Area Transit Authority site.
The Metrorail has been very good to me. I've never taken the bus in DC.
And there're always the taxis.
Good luck! I'm sure you'll get there. Maybe someone at the facility where you'll be training would like to go too, and you can get a ride.
Regards--
Mary Anne