An old towpath runs along the canals, good for warm ups or beginners. Off this there is a trail marked by yellow (I think) blazes. It's tight, twisty singletrack and fun to ride. No major climbs here but a couple smaller ones scattered around. As you work your way south in the park you'll hit Goat Hill with some climbing and downhill. The towpath and other doubletrack run pretty much north-south through the park. If you follow it past Goat Hill there's a road you can cross for more towpath but not much singletrack there.
From 495, take exit 21B toward Upton. Go for a bit into the center of town and the traffic light. Straight through the light onto Maple Ave. Follow until the end and turn right onto Pleasant St. Take that a few miles until you see Church St Extension on your right. Turn onto that and the parking lot is well marked and on your left.
http://www.singletracks.com/item.php?c=1&i=5227