This is what I've found to be the best combination of singletrack around Horn Pond. Although it's not the longest ride in MA, the singletrack here is relatively fast and flowy in sections and a little more technical in others. Definitely a great place for a quick loop after work!
From the smaller of Horn Pond's parking lots, head west along the road until you come to the access gate on your left. Follow the paved path down towards the water, then hang a right onto the first dirt path on your right. Follow this around the pond and take the first trail to your right. Pop back onto the main dirt trail and follow it across the smaller of 2 bridges you'll come to, then over a second bridge before charging up the hill on the other side of the 4 way intersection with a wooden post labeled George Olsen Memorial Trail.
After the climb, the trail will wind its way back down the other side. Make sure you stay left whenever you see the golf course, and right at all the other intersections. After crossing a 3rd bridge and exiting the woods onto the paved walkway near Lions Park, take a right and follow the paved path straight until it changes into a smaller dirt path. Hang a right when you get to the T intersection of another paved path, then downshift to your granny gear for the next right, which will take you up the steep section labeled "Nolan's Trail" on a tree.
At the top of the hill, be sure to check out the panorama of Boston's skyline on the left before following the path to the right. Watch out for a small cement block on the left side of the path, as it marks the start of the next singletrack section back down. Take a left here, and follow the trail down until you get to another T intersection. Take a right and follow the trail to a healthcare buildings parking lot, which you'll cross before hopping onto a wide gravel trail that will lead you back down to the paved path you were on earlier.
Hang a left from here to follow your steps back to Lions Park, then jump back into the woods on the left where you popped out. Take the first right up the hill, then follow the trail back to a 4-way section of singletrack with a wooden post labeled 15 where you had taken a right at earlier. Go straight across the intersection and backtrack for a bit along the same path you rode earlier before continuing straight instead of taking the trail to the left where you came from earlier.
Follow the path downhill through an open section of pine trees before coming to another T intersection with a wide sandy path. Follow the path to the left, and back to the 4-way intersection labeled George Olsen Memorial Trail that you passed through earlier. Hang a right on the same path near the 2nd bridge that you rode over earlier. Follow the dirt path back to the paved path, then the remaining few hundred yards back to the parking lot.