Here are your best N. VA options:
- Fountainhead -- the most demanding of the standard N. VA routes, but really not that tough; relatively well-marked; fun
- Wakefield -- flowy, well-designed; if you're going to be in town on one of the next Wednesdays, show up for the evening race --- good competition, low key (BTW, the place is small enough that even if you get a bit turned around, just ask someone to point you back to the parking lot and you'll be fine)
- Reston / Skatequest / Lake Fairfax -- this is my backyard stomping ground; a fun 10-mile loop can be done by linking the main trail from Skatequest to Rt. 7 to the Cross-County Trail to the W&OD trail back to Skatequest; show up on most weekday evenings in the Skatequest parking lot to get more info and/or riding buddies
- Laurel Hill -- this former grounds of an old prison is very flowy and fast; no climbs; well-designed; only two areas with anything that might be called "technical"
All of these are within 30 minutes or so of your location.
Have fun!