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I think it depends upon how much stopping and trips away from the canals you want to do. The canals while pretty in and of themselves, IMHO do tend to get a bit monotonous (excluding the lovely lock-keepers cottages) after the initial high of having the place to yourself/selves. As MisterMikkel said, 50-70km a day should be easily doable given how flat the canals are. I would have thought 70-120km as a descent stretch goal given how flat it'll be and no stopping for route finding required.
My advice would be to assume 50-70 the first few days and then when you get a feel for it on the ground, you can adjust the distance upwards if you are arriving too early...
Question for the hive mind on distances. I've not done any touring before and I'm looking at a multi-day ride along canals in France. Most of the itineraries I've seen are clearly aimed at bike hire / non-cyclists -i.e.: 21 km a day. They are great for planning stopping points but otherwise useless.
For a flat terrain and sedate pace what should I plan for each leg? ~50km a day? 70km? I don't want to undershoot and end up arriving at booked accommodation at 2pm everyday. Equally, I don't want to skip past stuff because I have too much distance to travel. Thoughts?