The Capital Crescent seemed like a nice trail, from my recollection (I'm from Philly, so am not intimately familiar with it, but have ridden it). You are riding across the country and you are worried about "a few miles of crushed stone"? ;-)
I liked the Crescent trail a lot more than Rock Creek, personally.
As I and CCrew said, seems like the most direct route is the Crescent trail to Georgetown, Key Bridge to Custis trail (which is generally not up to the standards of the other trails in the region, just so you know... was lots of fun when we rode it on a tandem with front/rear panniers pulling our son in a Burley trailer!), and then onto W&OD.
Rt. 15 can be busy, but shouldn't be quite as bad on a weekend.