DR FJL - If I'm remembering properly, the Courtyard Marriott Convention Center is one or two blocks away from the Convention Center. The
old Convention Center - so about four or five blocks from the new Convention Center. However, it is right around the corner from Ford's Theatre, and it used to be the Riggs Bank Building.
As for Metro access, you'd be a few blocks away from a couple of different Metro Stations: Chinatown/Gallery Place, Metro Center and Navy Memorial. As for access to Union Station, from Metro Center it's three or four short stops away. *Metro Center has a couple of different exit and entry points, so be sure to keep track of the most convenient exit to the hotel. Although, from the Chinatown/Gallery Place station, it's just one block away - if you get out on the Gallery Place side.
Here's the link to hotel on Marriott's site:
Washington DC Convention Center Courtyard MarriottAnd here's the link to DC's Metro system:
Metro - Opening DoorsAny other questions, just ask, or give me a call.
*Now, you do know/remember that Signature is not Metro accessible? And the cab ride from DC to Shirlington will not exactly be cheap either - and a lot of the cabbies have some major 'tude about crossing the river into Virginia (or Maryland). *And just make sure to arrange for the cab ride back into DC ahead of time too - although, I think Signature does offer call ahead cab cards in the lobby.