Myself and a friend actually tried this a few years ago in a very tight tunnel in Northampton. We obviously had an engine, but wanted to try to see how feasible legging was. It is super hard. Constantly balancing yourself on the board so you don't go in the drink. The actual work your legs have to do is nearly impossible if you let the narrow boat slow down, and the slippery and slimy walls add to all of that. I was covered in dirt and grime after a few minutes.

I suspect they probably used the barge poles to help too, but legging was definitely a thing. Like most of the jobs around the canal industry of the 18th and 19th century, it was incredibly tough and dirty work. I take my hat off to the the guys who did it day in and day out for very little compensation.