If you are looking for a place to run in the North suburbs of Boston, the Bruce Freeman Rail Trail is a beautiful shaded well-maintained rail trail. It has slight elevation at some stretches and runs ~11mi from Lowell through Chelmsford, Westford, Carlisle and Acton. There are many entry points with plenty of parking, see map for more details. Some lots even have porta-potties, should the need arise :) My favorite stretch is ~6mi from Chelmsford Center to Acton. The Heart Pond in Chelmsford is a beautiful spot to finish a run and enjoy sunset over the pond.
This is a popular trail, always filled with runners, bicyclists, skateboarders, walkers and their dogs! It’s perfectly safe from dawn to dusk due to its popularity.