With a little planning, an ambitious hiker could trek from the glistening waters of Kingston Bay on the South Shore to Plum Island on the North Shore, curving around Boston for 200 miles, and passing Thoreau’s Walden Pond in Concord along the way.  

The Bay Circuit Trail, a generations-old vision of connected recreational walking paths, is nearly complete, though its future grew uncertain in recent years with the impending retirement — from an unpaid post that had been his passion for more than 20 years — of 80-year-old Alan French of Andover.