To make the walk go faster, you can also rent a Capital Bikeshare bike in Crystal City:
http://capitalbikeshare.com/station_map

Take the Mount Vernon Trail spur up to the Crystal City Water Garden and turn left; there's a bikeshare rack within a block. Do keep in mind the pricing scheme for bikeshare penalizes long trips and rewards short, chained trips. The bikes have a front rack that can hold a smallish backpack or briefcase ("personal item" in flight attendant parlance).