Yeah, and to be fair, the 45-mile Newburyport trip took a lot more out of me than it should have (dcltdw's assertion that it was "all downhill" notwithstanding). Probably my next pick off that list would be the Ayer, Groton, Carlisle trip; the Nashua River Rail Trail north of Ayer is a lot like the Minuteman aside from being much less crowded and more consistently uphill, though not at a terribly steep grade. It's about the right length, but I really have no idea what's in between Carlisle and Groton.