Paul from Pittsburgh on 10/27/2013 2:52:15 PM:
Jeff, I just did this on my trip this month, although I rented a car and returned to Pittsburgh. Most rental car agencies only do one way rentals to major airport locations, but since I live near the Pittsburgh airport, this was convenient for me. I used Dollar, they had a location that was a few blocks off site of Reagan International. I stayed at a hotel in Arlington which was an easy 7 mile ride entirely on the Custis/Mt Vernon Trails until I got near the Dollar location. Then it was just about 3 blocks on roads, pretty easy and safe.
I think your only option to return to Cumberland is Hertz. I checked into this because of the govt shutdown, in case I could not continue past Cumberland, and Hertz was the only option I found that had a location in Cumberland AND did one way rentals. I think their Arlington location is on airport premises. I don't imagine it would be a problem at all to bike there, since the Mt Vernon Trail literally goes right to the airport entrance.