I would recommend the Canal Du Midi linking the Med. to the Atlantic. If you do a search of of Canal Du Midi and cycling you will find tour operators. To me they seem a bit pricey. Try organising it yourself, the route is easily followed and you can figure the distances fairly easily and book online. I have never cycled the canal but have been all around there and think it is more variable than the Loire and though touristed it seems to me at least it is less touristy. The exception is Carcassone which is touristed but well worth the visit.