Check with Loudoun County Transit. They have commuter busses that run to Purcellville, although I believe that they don’t have any incoming to DC in the afternoon. They might however as that’s where their depot is.
They’re full coaches, no racks on the front but they do have storage for bikes underneath.
Other option is to double back to Reston, pick up a 505/555 bus out of the Sunset Hills Park and Ride and hook to WFC metro. the P&Ride is at the 18mi marker which means 27 miles back from Purcellville instead of 45.