We did make it around the 4.1 miles, but found it difficult to find the right trails because they were marked very poorly. At one point, the trail marker we were following was replaced by a different color one, making it seem like we took a wrong turn. This ended up sending us about .7 miles out of our way. Other than that it was a good hike, with nice views of the Sudbury River and the Fairhaven Bay. We crossed the commuter rail and had a picnic on Walden Pond. Easy to find the parking area using the description on this website.
Wright Woods is a hidden spot right behind Walden Pond. While Walden Pond can get inundated by locals and tourists to the point where the parking lot shuts down in the summer, most people come for the pond and don't even know about the back trails - just cross the Commuter Rail tracks and you're in a secluded pine forest suitable for snowshoeing, horse riding, running, cross-country skiing... You can walk down to Fairhaven Bay, a very wide section of Concord River which looks like a lake. The area connects to Adams Woods and Mt. Misery to the south. There are many trails in the area, so you can loop around to make your activity as long as you wish.