Personally I find a noticeable difference between 10F and 20F when fitness walking or snowshoeing- especially if there's a wind. I don't usually attempt to bike below 25F --unless it's sunny, no wind, and dry roads, in which case I might try a short ride.

My main worry for you is not so much the cold-- but if your tires slip on snow or ice and you then skid into the path of a car. I assume the road shoulders there in MA are piled with snow drifts and crap right now, like they are here in my area of NY- that makes for more dangerous riding when you have to ride right in the lane on the edge, near the unpredictable dirty ice patches. Do you at least have studded tires?

My personal limit is to not ride when it becomes really unpleasant or dangerous, regardless of whether I'm trying to achieve a personal riding goal or not.
Be safe out there now!