Another thing to remember is that if the towpath turns out to be in bad condition due to rain as you approach the DC area, you can (depending on where in the DC area you live) get off the towpath and take White's Ferry across the Potomac. From there, it's less than 5 miles to Leesburg and the paved W&OD Trail which will get you to Arlington and DC (with a connection to the Custis Trail if you're going to DC proper). Route 15 in VA to get into Leesburg has lots of high-speed traffic but it also has a wide paved shoulder to ride on.