Reston is a great place to launch a bike ride!
Given your average pace, the MUPs aren't the best place for you. The W&OD Trail will be lightly traveled on the weekdays, and should be okay for some faster riding and as a way to get to Leesburg (about 10 miles west/northwest of Reston, and the end of the suburban sprawl to the west of DC). If you take the W&OD toward DC, it gets a bit more used, as it is a commuter and recreational path for the greater DC/NoVA area.
Click
this link for a cue sheet and starting point for a 40 mile ride that starts at the Reston Parks & Rec Center. It's provided by the Potomac Pedalers Touring Club, one of the big clubs in the area. They also have a lot of cues that start out in Leesburg. The PPTC cue sheets are well-tested routes.
And it's worth dropping a line to the
Reston Bike Club - you'll be in their stomping ground, after all. They have a series of
weekly rides that just started up this week with the return of DST.