It will all depend on the time of day that you are traveling. You can take the VRE or the metro from either Springfield or Van Dorn. If you are driving then most likely you are going to hit I-95 N depending on your exact location (either inside Ft Belvoir or a community outside) will determine the best route for you to take. If you are driving alone during rush hour you can anticipate about 45 mintues to drive. If you are an early commuter and can hit the HOV lanes that time will lower considerably to closer to 20-25 minutes.
You could also SLUG again depending on your location.

Sorry I can't be more specific but location make a big difference in your options.