If you're looking for a scenic 50-60 mile bike route in No. Va

I highly recommend this one:

http://www.bikewashington.org/routes/bluerdg/index.htm

From Manassas area its barely 30 min drive. Prob an hour from DC? 

Minimal traffic (except crossing a couple of highways (7 & 50); smooth roads; & nice scenery).   A couple of the hills really kicked my a$$, but if you wanted to do a 50 mile without hills you could just turn around at Bluemont.