Originally Posted by Mtn Mike
.....you will find that Route 123 (which runs right by GMU) has an MUP that runs from GMU south, almost to I-95. After you cross the Occoquan river on 123 you could jump onto side roads and head south, but I'm not sure the names of the roads....
I can supply the rest.
123, is ok, but it is the long way. You can take Old Bridge and the PW Parkway to Manassas, but you might as well go straight to Manassas and save about 12 miles.
Here's the route I use, with approximate mileage
West on Braddock using side path on south side (0)
cross Centreville Rd / route 28, left on Old Centreville Rd (7)
left on Compton, quick right on Ordway (8.5)
cross Bull Run (hill) turns back onto Old Centreville
right on Polk, quick left on Manassas Drive (11)
right on Mathis (11.5) this is a commercial area, so watch for traffic
left at T on Main (13) in downtown Manassas
cross under railroad tracks, right on Prince William (the visitor center at the train station has toilets and water)
left on Wellington (14)
right on Clover Hill (15)
left on Hastings (15.5)
right on Lucasville (16)
right on Bristow Rd (19) narrow and busy so watch for traffic
(toilets at the historic district office, if it's open, and your last chance for a long while without a detour off the route.)
left on Old church (19.5), right on Crockett (20.5), left on Flory
right on Parkgate (21.5), left on Fleetwood (22)
at Sowego bear left on Brent Town Rd / 612 (29)
(if you need a toilet, after about a half mile on Brent Town go right on Courthouse Rd, after about a mile there is a store on the right that usually has a portajohn (may be seasonal))
cross Aquia / Garrisonville Rd (34.5) 612 turns into Tacketts Mill
bear left staying on Tacketts Mill (36.5)
left on Poplar (37.5)
left on Truslow (45)
about a mile after crossing under I-95, you'll reach route 1
go south on route 1 (watch the traffic) past the light at route 17
next right on West Cambridge and left under route 1 onto River Road.
(I've never taken River Road, but I'm pretty sure it goes through.)
right on King's Highway, over bridge into Fredricksburg.
The town is a maze of one way streets, so you're on your own from here. Just keep heading west and up hill.
Going this route to Manassas and around west of Quantico is a nice ride on many quiet country roads, and except for a few stream crossings it's not too hilly (worst is probably on Braddock Road, which is more up than down)
ps Here's a gmap I did for someone last year. The beginning and end are a little different, but most of it is ok.