The Cotton valley rail trail is a 12 mile trail between Wolfeboro and Wakefield that is used for walking and biking. It's an easy trail, mostly flat and hard packed gravel and has some nice scenery along it. One thing to note is there can be a lot of walkers including children along it so if you are biking expect some stops and starts as you try to pass them. Overall, it's a very nice easy trail. There is parking in Wolfeboro for it behind the Wolfeboro train station.
http://cottonvalleyrailtrail.org/