Massachusetts Bay Circuit Trail. This trail circles Boston for about 200 miles or so, passing through several state parks, forests and reservations. It is managed by the Appalachian Mountain Club (AMC.) As of this moment (Jan. 2015) I have walked about 25 or 30 miles of it, mostly from Billerica to Wayland MA.
5/24/2014: Orchard Road (near Bedford/Billerica line) to Rte. 62
5/31/2014: Rte. 62 to Concord Center
6/1/14: Concord Center to Rte. 2
6/10/14: Rte. 2 to Wayland (near Rte. 126, a few miles shy of Rte. 20)
11/11/15: Section 9 near Medfield, with IMScotty