Marked loop around the lake, all told about 6-7 miles depending on if you follow the loop directly. Singletrack branches off here and there which could add some miles to your ride. Sometimes it just braids around worn out parts, other times it's a small loop of its own. One side of Whitehall is pretty fun, the other is... well... interesting. Much of what you expect from a lake loop with mud, roots, rocks, and tight trails in patches. Some hills too. Eventually the trail hits a road and if you ride across it you'll find trail linking to Upton State Park. A trail near the parking area can link you to Hopkinton State Park. String em together and you have potential for serious miles
From 495, take exit 21 toward Hopkinton. Go by the pond and turn left onto 135 after you come up a hill. Follow for a few miles until you see the signed parking lot on your left
http://www.singletracks.com/item.php?c=1&i=7140