At lunch today the trees looked pretty pink from afar, so they may still be okay on Sunday, unless we get rain or wind in the interim.

The walk is do-able, but it'll take a while (check the distance). Also, I don't know the mechanics of getting from the path up to the 14th St. Bridge. That might require a substantial detour. As an alternative plan, you could take the Yellow Line to L'Enfant Plaza and walk back.