Well I did that considerably faster than 5 mph on a Brompton on the section from the Garonne to past Carcassonne. But then I had a Brooks saddle so no pain for me.
That's the point you can't beat a Brooks if your crutch fits. Worth every penny for the pleasure. If it doesn't then its going to be hell. Which points not so much to finding a comfortable combination for you well in advance of your trip so you aren't going to worry about the third and fourth day. A tour is not the time to experiment. Plus it can take a Brooks saddle several months to mould your buttocks into the right shape [some deluded people think its the other way round!].
Canals are, by definition flat, so you don't have to worry much about weight. I've never yet filled two panniers even with a laptop. Working out what you need to take and, more importantly, what you don't need is worth researching. Also to keep a record of what you use so you can go with even less next time.
Enjoy. Then maybe you can write up and enjoyable report to seduce some of us across the pond to try something new. Though personally that won't be before 2021