The list in this comment is pretty good. I ride musquash and masabesic regularly and have been to all of the others mentioned. I actually think there’s a ton of trails around. Just get Trailforks and explore around the map!

I think when you get up to the Lincoln area it starts to lack, but southern NH has a lot going on for trails. And, sometimes I’ll even head down to mass too (eg to the trail system around Billerica that I’m forgetting the name of).