When there is no snow, another lovely trail for running in the North suburbs of Boston is the Nashua River Rail trail. It runs ~12mi from Ayer MA to Nashua NH. The trail has some elevation at some stretches especially between Pepperell towards Groton. The path is very popular with bicyclists and some stretches especially through Groton by the pond is popular with parents with young kids, walkers and their pets.
There’s plenty of parking at Ayer, Groton, Pepperell and Nashua. See map for more details. The trail runs by the Nashua river in Pepperell and the Rail Trail ice cream store just by the trail entrance in Pepperell, is a local favorite. The trail is well-trafficked dawn to dusk only between Ayer and Groton center and from Pepperell to Nashua. The full trail is usually busy only on weekends, with plenty of bicyclists. Its best to follow rules and keep right when running on this trail and keep a ear out for bicyclists calling "on your left"!
My favorite section is ~1.65mi stretch between Groton Center (Station Ave) towards Ayer, which runs by Groton School pond filled with fish, turtles and beautiful birds. You can even see people fishing here.