There are aprox 40 miles of ride-able trails inside the park boundaries made up of about 50 % single track and 50% service roads. The park straddles an area around two different mountains with a large lake in between them so expect some climbing but also anticipate some great downhill runs too. With a road leading all the way to the top of Mt. Constitution a shuttle for the downhill runs is also possible. Because of the popularity of the park in the summer some trails, mainly singletrack, are only open Sept 15 thru May 15, all the trails are very well signed and its very clear which trails are open to which users. There are plenty of trails open to us year-round.
I parked at the Cascade Falls trailhead and linked together a few trails that took me on a 10 mile ride up and over Mt. Picket (mostly service road), down the north side of the mountain and returned south along the eastern edge of Mountain Lake (all single track). Pick up a trail map and you can link together many different routes in this area.
There are 3 different campgrounds in the park with at least one open year round, also many hotels, resorts and B&Bs on the Island.