I mapped out a route for a friend of a friend riding from DC to Williamsburg back in April. He said it worked well for biking. Here is the last leg of it, Ashland to W'burg.
http://www.gmap-pedometer.com/?r=2671730
If you can get to West Point you can follow it in reverse. Route 30 might also work.
Can't help with Ashland to C'ville, unless you want to ride by way of Warrenton and Culpepper.