Yeah, Fairfax Pkwy (7100) runs near Manassas but the ACA in that area is southwest of Manassas, near Nokesville (corner of 646 and Fleetwood). 7100 would run too far to the east, requiring that I go 20 miles east and then cut 20 miles back west. It looks much easier to go almost strait south from Leesburg to Nokesville. The best road by car would be Rt. 15 from White's Ferry strait south to Gainesville, then south to Nokesville on 619. But Rt. 15 the whole way doesn't look bike friendly.
I'd be interested to here from anyone who has tried Rt 15 from Leesburg south.