This should give an idea of the route.
I do it a little differently. I use the Metropolitan Branch Trail from my house to the Mall, cut across the mall, follow the river up and onto the CCT. On the way back I cut over to the MBT. Easy enough to reverse it or change around if you are coming from someplace else.
Granted I have my winter hibernation weight so I suspect I will need to work up to it again.
My old regular loop was starting in Shirlington on the 4 mile run trail, continue west to the Custis trail, take that over the roller coaster ride down along 66 to Roosevelt Island, then Mount Vernon Trail back to the start of the 4 Mile Run trail just past National Airport.