Koffee,
You'll probably need a car to get to any good starting points. I used to live in NoVa and did a lot of riding with PPTC. My advice would be to check their website:
www.bikepptc.org.
We used to start a lot of rides at the Tyler Elementary School on Rt 55 in Gainesville but this may be all developed now. Rt. 55 beyond Rt. 15 probably isn't too bad. It parallels Rt 66 so it doesn't get too much traffic. From The Plains you can head north to Middleburg and through VA Hunt country for very pretty ride. I think there are still good rides out of Nokesville which is west of Manassas.