Hi there, fellow Boston area cyclist here.
The best (and unforgettable) *truly *long ride is an all day affair, bike to Provincetown (tip of Cape Cod) from Boston (or somewhere outside of Boston) and take the ferry back that evening.
More reasonable long routes include going up and around Cape Ann (Gloucester, Rockport), out west to Wachusett mountain, or any of the local club weekend rides (Charles River wheelers has plenty of routes)
You can easily rack up 50+ miles just going up and around the near suburbs of Boston, such as Concord, Bedford, and Sudbury/Lincoln. These are all heavily biked towns with lots of wooded roads, strava segments, and flybys from all sorts of rippers to keep you motivated. There’s almost always places to restock on water/food in the center of any given town.
I’ll post some of my strava routes down below from past rides, although I’m curious to grab whatever others post as well as I’ve been looking for some new routes for myself lately. Most of my rides start from Somerville so honestly in West Newton you’re already 10 miles closer to the “good stuff” in a lot of respects. Happy riding!