There are two parts to Callahan - the nice side, and the ugly side. The nice side is about an hour's worth of riding along great single track through the woods and across fields. A few roots and rocks, but otherwise, very nice trail. Watch out for the abundance of people/pets, though. Plenty of trails there - once you head downhill to the Pipeline, you'll find another entrance to the park (Edmands Road), which features a tough climb, leading into a network of very fun riding. Signage is limited but improving there, and you can easily add a few more miles of climbing and descending amidst the Marlboro and Sudbury-side of the trails.
Take exit 12 (Framingham/Route 9) off of the Mass Pike and head East on Route 9. Follow the exit off of Route 9 for Edgell Road (also for Framingham State College). Take a left towards Sudbury on Edgell Road. After about 1.5 miles, take a left onto Belknap Road. Go straight through an intersection or two until you turn right onto Millwood Street. The entrance to the park is on your left across from the golf course - plenty of parking if you get there early enough.
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