Probably too late to change your plans but it is a lot easier finding a place to park on the Pittsburgh end like my house. There is also the police station in Mckeesport, the parking lot in Boston, parking lot at the pump house. All free. Plus I would bet a lot of hotels would let you park here if you stayed your first and last night.
Be sure to check out this city while you are here. You should not miss Bicycle Heaven.
Try to find a warmshowers host in DC or why not just rent a car for the trip to DC or just drive here? A lot better than getting in at midnight.
Let me know if you need help at the Pittsburgh end like a lift back from the airport etc. And we even have a bike trail right from the airport.
Last edited by spinnaker; 05-18-15 at 08:24 PM.