The photo below is from a 1973 issue of National Geographic, which means it predates the Mt. Vernon Trail. The article is entitled "Bicycles are Back - and Booming!"
"Cyclists claim two out of four lanes in the eight miles between Alexandria, Virginia, and George Washington’s Mount Vernon.” Reads the caption.
I think this is the Mt. Vernon Parkway, but someone else thinks it's Route 1.
The article also mentions Marie Birnbaum, the U.S. Department of Transportation’s bicycling-program officer at the time. I recogonized the name, because someone of the same name was on the board at WalkDC not too long ago, and I would guess still lives in the DC area. Same person? She also contributed, along with WABA staffer Cary Shaw, to this classic 1974 EPA document on Bicycle Transportation.
You might be able to read the whole NG article here.