I can't believe I overlooked this useful coincidence for so long. The distance from Hurd Field to Mill Street, one and three-quarter miles, is also the distance from Alewife to Mill Street. If you do as I do most days, entering the trail at Hurd, going east to Mill Street and then turning around, the outbound milestones show your total distance -- no computation needed until you get to Bedford and turn around again.
Today: 16.5 miles. This year: 293.3 miles.