I live 2 miles from Jamestown Island, where the Capital Trail intersects the Colonial Parkway. It is a great trail for road biking. It runs right by my neighborhood and I ride it all the time.
As of now the trail is paved from the Chickahominy River bridge on Rt. 5 to the Colonial Parkway (maybe 5 miles), and from the CP you can ride all the way to Yorktown (maybe 25 miles). There are some short sections between the Chickahominy River and Charles City courthouse that are paved (but nothing to drive to get to). So eventually you’ll be able to ride all the way from Richmond to Yorktown on a beautifully paved trail.
Like you said, definitely not a mountain bike trail, but a good way to get in shape for some offroad riding.