Anywhere with canal towpaths - for example the Brest - Nantes canal which runs for more than 200 miles across Brittany. It's the first part of the Velodyssee route which continues south down the west coast of France as far as the Spanish border. Lots of info and blogs on line.
The Loire valley is in the Northern half of France and is absolutely full of excellent, well signposted cycle routes. Also plenty of campsites. About 4 to 5 hours from Calais.
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Agree with Colinlob. The Loire area is excellent for safe cycling routes and many campsites in the region will have well mapped routes for you. An excellent site with this in mind is Les Saules at Cheverny. Take a look you won’t be disappointed
------------- Never worry about life - you’ll never get out of it alive
Don't forget the Loir VAlley. You may at first think I am repeating what others have recommended, but actually note there is no "e" at the end of Loir. This valley is a little north of the Loire.
We visited a few years ago, staying at La Fleche. Plenty of lovely cycling from there. The area is referred to as a well kept travel secret in France.
There is a network of old rail routes that is easily accessible from the town.
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Check out the Voie Vertes. Many, if not all, are old disused, but paved, railway lines and are therefore not very hilly. I would also agree with the comments about canal towpaths, we use them regularly all over France.