If you do take the Minuteman Path to the end, the dirt path that continues definitely needs a mountain bike. It is very muddy. I would recommend getting to Cambridge, get on the extension of the Minuteman Path that starts in Cambridge and take that to Bedford. Then, you can ride into Concord on the road, on Rt. 62 into Concord Center.
Riding in Boston is scary and unless you are used to city riding, I would say stick to the paths, until you get at least to Arlington. I live in Concord and would be happy to help you in any way.
There shouldn't really be any hills on this ride. maybe a few rollers.

Robyn