Probably Littleton? You can get most of the way there on non-death roads (the bit through Littleton Center isn't great, but it's slow at least).
Life goal for later this summer/early fall is to scout the bit of the 100k route
I haven't yet scouted for interested parties. Also interesting: the Bruce Freeman Rail Trail almost extends all the way from Lowell to Concord, which would be a fun train excursion. If they can ever get the network of trails built out (that, Assabet River, Wayside, Reformatory) there would be some excellent, if flat, long loops.