Could drive (the wrong direction, granted, away from DC) to Brunswick to get the MARC. More choices of train in both directions. Although, glancing at the schedule, I guess there isn't one that's much later in the morning. It leaves at 7:40am. The drive could eat up the difference depending upon where in Frederick. Also with MARC overall you have less choice of when to leave if you want to stay into the evening; they don't run that late on the Brunswick line, 7:15pm last out.

Driving in DC sucks, IMO, but if you're going to do it: don't leave until 9 or so. Take 270 to 495 towards Virginia. Take Cabin John Parkway exit. Bear left and take Clara Barton Parkway into DC. This takes you to the edge of G-Town, west end, and near the Foggy Bottom area, which isn't far from where you want to go I suppose. Can't tell you where to park, and it could be difficult in mid-day when you're not checking into a hotel. But that parkway is by far the best experience I've had driving in from the 270 side. It'll probably be faster than the Metro or train. Metro from Shady Grove takes forever, like an hour I think. Driving at non-rush from that point takes less time methinks, although it can be much more aggravating.