This was quite intriguing, SFA...completed 1789 and 3800 yards long (per WikiP)...which is a bit over two miles!
I learned they got the boats through these tunnels, including Sapperton, by 'legging' them -- "Prior to the introduction of motorised boats, legging was one of the few options for getting a boat through such a tunnel.Two people were required. They would lie on a plank across the bows of the boat, and holding the plank with their hands, would propel the boat with their feet against the tunnel wall. This was quite a dangerous activity and resulted in many deaths."
And this tunnel was passable until at least 1966!