I second Cycle Loft. They are big enough to provide you with a lot of choice. There is an OK shop in Chelmsford, on Rt. 110, but it's small.

You should check out Bike Rides for Ordinary People... the leader, Bruce does rides at a very moderate pace. Most are closer to Boston, though. Also, AMC has "easier" level rides on Tuesday nights through the summer. They leave from various points in the western suburbs each week. Check out www.outdoors.org, go to the Boston chapter and then click on bicycling.