I don't know how much of a help I will be, but this shows the route I normally take to get from Mount Vernon to Occoquan combined with what I would probably take to get from Occoquan to Prince William Forest:
http://goo.gl/maps/dhLfk I've had pretty good experiences riding on Lorton Road and Ox Road, but past Occoquan I'm in the dark as to a good route. I have no idea what Minnieville Road is like (it is shown as a preferred bike route in Google Maps), but Spriggs Road and Dumfries Road look feasible for that stretch.
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