GW Parkway to Route-123 to the Toll Road to Reston Parkway to 606. 25 miles maybe? But almost all of it without major traffic. Problem will be how do you get out of the city at rush hour. Depends on where in town you live. There is bus service (Fairfax Connector) to Herndon from the West Falls Church Metro station, but whether even to consider it would depend on where in Herndon the job is.
Bottom Line: Might be a lot easier to live somewhere near Herndon and commute into DC when you happen to want to be there...