Depending on where exactly you are working downtown, Reston to DC should take about 1:15, door to door, from a park and ride (home to P&R, bus to WFC, metro to your office, walking time).
There is one express Metrobus that goes from Herndon-Monroe P&R to L'Enfant. It's the 5A on this page:
http://wmata.com/bus/timetables/timetables-state.cfm?State=VA. (BTW, Firefox chokes when I try to open the timetables--I use Chrome.)