20.3 Miles Easy
The Minuteman Commuter Bikeway Trail is great for those wanting to get in and out of the Boston area by bike or by foot. The entire trip itself is 20.3-miles if you plan the entire out-and-back full length trek, otherwise the path stretches 10 miles one-way, but there are a lot of extensions that lead off from it. It is quite well traveled so expect traffic - bikes, pedestrians, etc
Multiple bike shops, repair places, and shops to get drinks and food along the way.