FWIW, google maps claims it's 52.2 miles from DC to Purcellville, and 38.1 miles from Purcellville to Front Royal. Those are estimates for drivers on roads, not cyclists on trails. It's 9% less than 2/3 but well over 1/2, but I agree that I did indeed overestimate (a SWAG, actually) the percentage. Nevertheless, I would still recommend taking the W&OD all or most of the way toward Purcellville if I wanted to get to Front Royal from DC by bike. Even following google's 2 road routes DC->Purcellville->Front Royal, it's only 20 miles longer than the most direct road from DC->Front Royal, which is I-66. Obviously a cyclist leaving DC is going to have to take something longer and much less direct than I-66, so going via Purcellville wouldn't necessarily add that much mileage. It's certainly a pleasant trail most of the way. Having said that, I have no idea if there's a decent way to bike from Purcellville to Front Royal. If route 340 north of Front Royal is anything like route 340 south of Front Royal, I sure as hell wouldn't recommend that road for biking!

Back to Harutz: If you're approaching DC from the west or northwest, the Capital Crescent Trail is a very nice way to bike into the city. If you're coming from the north, DC has plenty of residential streets which are nice for cycling. And if you arrive on the weekend, Beach Drive in Rock Creek Park is largely closed to motor vehicles.