Actually have you been to the Nashua River rail trail? It goes through Ayer, Groton, Pepperell and Dunstable. It's only 11 miles long, but it's great for time trialing or leisurely flat rides. I often ride it with my uncle and his gf or my brother.

http://www.mass.gov/dcr/parks/northeast/nash.htm

It would probably be a 20 minute drive to get there for you, but it's lovely and totally worth it!