There are some trails specific to mountain biking that were built by the Friends of Massabesic Mountain Bike Association, but that group has nearly all but dissolved and the trails are growing up last I knew and some have become unusable.
The MTB club is useless and revitalizing it seems to be not on the list of priorities right now.
The area is the reserved land around the Massabesic Lake watershed which includes Massabesic Lake, Little Massabesic Lake, Clay Pond, Tower Hill Pond and other minor water bodies within over a thousand acres of protected woodland owned by Manchester Water Works, a private utility for the treatment and provision of drinking water for Manchester and some surrounding towns.
An old railroad bed cuts through the conservation area and provides for riding on various terrain from cinder bed to dirt to gravel.
A snowmobile club also has cut many trails through the woods off the rail trails that it maintains. The snowmobile club is very active and keeps up on the trails. Unfortunately, since there is no active bike club, the features of the various trails are all natural.
But one can travel a long distance, either with a gravel grinder on the rail trail or go off trail to discover a beautiful bucolic natural and quiet landscape beyond.
Sometimes the trails end in a subdivision. Most of the trails cross country roads and signage is limited. Signage for the trails is also limited to reference to a map made up by the snowmobile club for their use.
Topo maps are useless as they are usually old and don't show any of these trails. Many of the snowmobile trails are old fire roads or logging roads, but there are some nice single track offshoots as well, either built by the MTB club in the past or by rogue trail builders.
During the spring, summer and fall when the lake front park is open the city provides portable toilets.
People are forbidden to swim or even put their feet or hands in the water. Bring your own drinking water and provisions. Bring provisions with you that you'll need in case you get lost, including a road map of the area so you will know your location from roads you cross. If spend anytime out there you can be out pretty deep. Don't forget your bug repellent and tire repair/air kits!
The park entrance closes at 8pm with a gate and police do patrol the area. Parking on Depot Road avoids this inconvenience.
Beware of hunting season also as you get further out into neighboring rural towns.
Overnight camping and fires are not allowed by the MWW and I'm told they patrol the area by firetowers and ATV's. Fines are stiff.