Hmm. Google Maps thinks that it's 25 miles from the end of one bike path to the end of the other, via US-3, which is reasonably direct. I make no promises on difficulty, beyond that the route I'm eyeing has no hill arrows on the bike map (and wow Westford Center is at the top of a hill). But beyond that, that assessment sounds about right (NRRT ~= 10 mi, + 25 mi, + Minuteman). Funny how, when you've done it enough times, making the Minuteman your last ten miles feels like you're "almost home"...