While relatively flat, the Norwottuck Rail Trail covers 22 miles out-and-back starting in Northampton, MA, passing through Hadley, Amherst, and South Amherst.
The start crosses the beautiful rail bridge in the Connecticut River Greenway State Park, with views of the Seven Sisters mountain range to the south throughout the trail. It's quiet, paved the whole way, and very enjoyable place to be.
More details on the trail are available from the Commonwealth of Massachusetts: https://www.mass.gov/locations/norwottuck-rail-trail.
I just noticed this one. But in my mind it makes more sense to have it start and stop at the J. Elwell Conservation area, and leave out crossing the busy Damon Rd twice. In that case Ben Groleau is probably the fastest on Strava, but their might be older even faster known times...