You guys should try out Fountainhead Park in Lorton. It's a very nice, well maintained trail. Has nice singletrack riding, and a cool downhill part near the end. It's off 123 just past the Lorton prison coming from woodbridge, I think there's directions on mtbr.com

Also, if you wanna drive a bit more, Hollywood Farms in Fredericksburg is a cool trail tool. Lots of tight singletrack, it's on a farm off rt 3 down there