I can't remember which one we stayed in last year (it was either a Marriott or a Holiday Inn) but it was close to the start and finish which was convenient. It seems like there were a fair number within walking distance. Wherever you stay, I think the publice transportation to and from the start/finish is pretty good.

Well, we stayed all the way in Springfield but had no problem getting to/from the starting line, the public transportation made it so easy to get around w/o a car.

I've stayed in downtown DC for all of my area races, including MCM and Army Ten Miler (which starts in the same area). That's largely because I like the extra excitement of a city and being within walking distance of all the fun touristy stuff. If you don't care too much about that, then yeah, try to stay close to the start. The metro will certainly be crowded, but it didn't really bother me and I'm claustrophobic. Getting to baggage after MCM was a whole hell of a lot worse than the metro station! At the closest station, I think I got through in 20-30 minutes, not tragic. It's also not a bad idea to walk to one of the alternate stations; post-marathon walking is crucial and it's less crowded. Note that I ran MCM two years ago, not last year like most here, so things could've varied some.