Whitehall and Upton State Parks in Eastern Massachusets have some of best singletrack in the region. I mapped out a route combining the trails in both parks that is about 20 miles, which is a long loop considering the average trail length in the area. The trails are not hilly, but they are winding to say the least.
The loop is technically a set of three loops. My suggested direction would be to start at Whitehall SP parking area, run clockwise around the Whitehall Reservoir to get to Upton SP, and then follow the direction of my GPS track in Upton. The Whitehall perimeter trail is marked in blue. When you reach the road, run up the road to your left about 100 meters to find the trail leading to Upton SP. The trail you are on (Whitehall Trail) is not on the Upton map, but will intersect with Whitehall Rd. on the map. The Upton section looks confusing, but it's bascially a perimeter of the park with some specific detours to hit trails in a certain direction. The Whistling Cave trail should really be run downhill.
The sequence of trails in Upton is:
Whitehall Road Grouse Mammoth Rock Old Hopkinton Rabbit Run CCC Way Middle Road Dean Pond Park Road Nuthatch Whistling Cave Loop Road back on the Whitehall Trail