Dan Bates submitted this logical, 45 mile loop:
Going clockwise from the North Windham Road parking area, this loops links the Nipmuck Trail's East Branch (6.4 miles), followed by 15.9 miles on the Nipmuck Main Trail to its junction with the Natchaug Trail, followed by the Natchaug Trail in its entirety (17.8 miles), followed by 0.25 miles on Potter Road to reach the Airline Trail, which is followed for ~5.5 miles to an exit near Boulevard Road, followed by a ~0.7 mile road connection back to the parking on North Windham Road.
Although there are a couple of possible entrances to the airline trail from Boulevard Road, bushwhacking and trespassing aren't allowed, and neither is running on route 6. Being a loop, you can start anywhere and go in either direction.
On paper, this adds up to over 46 miles. On my gps watch it came out to just about 45 miles exactly with 4550 feet of elevation gain/loss.