I lived in Reston until a couple of years ago and commuted in. I was surprised at the number of reverse commuters to Reston. I don't think getting to work will be much of a problem but the trip home could be a nightmare if you work any later than your stated 3:15. The spur to 66 and then 66 itself can be gridlocked on any particular afternoon. The traffic is so heavy the busses are allowed to legally drive down the shoulder.

Forget about the GW Parkway/123 route. You don't want to get into Tyson's Corner at any time of the day.