I agree with Noreaster, it's going to depend on the type of riding you want to do. Harold Parker is great and my primary riding area, but he is right about it being rocky and tough to carry a lot of speed. You have to be very technically sound in there. I'm not but I love the challenge. Russel Mill is in Chelmsford and is within 2 miles of Great Brook Farms. Both are great trail networks and are maintained by MV NEMBA and what a job they do! Russel Mill is a bit more technical then Great Brook, but they are both great ride. More flowy singletrack and tight switchbacks. They are not really that close to North Andover though. One place you might like to try is Willowdale in Ipswich. Great flowy singletrack not very technical, and you can fly! It is a lot of fun! I would take you out on a ride however my tennis season has started and I play on Saturdays. Hope you can find someone to lead you around. If you want poke into the North Shore NEMBA facebook page and post about a ride leader there. I'm sure someone will chime in. Good luck!