Dammit, my third day with new tires and new rear tube, and I arrive home to find a big old nail driven right through it. (Add another eight K to my distance today, riding to Caryn's in Watertown and back.) The route, though, is nice: there's a gravel path behind Alewife station that goes to Blanchard Rd near Belmont Centre. It's a bit hard to find the Alewife end of if, hidden in the greenery across the road from the main passenger pick-up point, but once you get in it's a peaceful little green tunnel with rabbits and and chipmunks and what might have been a jay.
Apparently, though, you do need to be a bit watchful for nails. Dammit.