Looks good up to mile 52. Recommend you stay off Centreville Road. Continue on New Braddock for a block to Old Centreville Road, takes you to Ordway at Compton.
Recommend you stay on Mathis and take Main to Prince William. Much less traffic and Old Town Manassas is worth a stop.
I've never ridden Lake Jackson Dr but it seems to be the long way around. For a shorter route, go West on Prince William, then left on Grant, right on Wellington, left on Hendley, right on Hastings and left on Lucasville. That takes you to Brentville. Watch out for Bristow Road, which has fast traffic and no shoulder.
West around Quatico is good (I've ridden most of it and it's pretty country).
I've never ridden Garrisonville Road, but I've driven it and I'm not sure I would want to bike it. Alternate I usually recommend and I've heard works really well is Thacket's Mill to Poplar, then just before 17 take Truslow to Fredricksburg.