Back when some of us went walking along the Capital Crescent Trail as a show of solidarity with the Avon walkers, a few of us were partial to the idea of walking from time to time. So... perhaps we could try that again!
My initial idea would simply be to start off outside the Rosslyn metro station, make our way onto the Mt Vernon trail heading south and call it quits when the group decides to. Provided we stop prior to or in Old Town Alexandria, we should have options for everyone as far as metro and possibly snacking are concerned.
I'm not married to either the trail or the date specified, so if there's a consensus to change either, by all means.