We did this three years ago. We started in Sete, walked the Canal du Midi through Beziers and Carcassonne to Toulouse, and then picked up the route from Arles. My husband is fascinated by locks and the canal workings, and I wanted to see the canal before all the beautiful trees are cut down.
I'd have to say that it does become rather tedious. I was incredibly glad to get onto the Arles route at Toulouse, and thrilled with the first small hill! The Arles route is lovely. One or at most two days on the canal was enough for me. But my husband enjoyed it immensely. We have friends who have since walked it and the wife also thought it a bit tedious.
The path beside the Canal is known and well used by cyclists and we were constantly having to leap out of their way. We did not meet any other walkers. The folks holidaying on the boats were very friendly and we kept meeting the same people - the boats would get ahead of us and then we'd catch them up as they waited to go through a lock. We had lots of offers of lifts, which we refused on principal, until nearly at Toulouse when we had a lift for about two kilometres for fun. It was, we enjoyed their champagne....
The route meanders and winds backwards and forwards in loops, as canals do, and you cover a lot more ground that it seems just looking at the map. You could no doubt shorten the distances considerably by some road walking but we didn't - being committed types. I can't recall now how many days it took us. We averaged about 20km a day. It interests me that we didn't find ourselves walking any further than we would have done on hills. The days seemed quite long enough.
You do need to plan accommodation a bit; it tends to be spaced at biking distances rather than walking distances. The places we stayed in were universally excellent, but not cheap. We stayed almost exclusively in chambre d'hĂ´tes, booking demi-pension and an evening meal whenever possible. Many of the villages are tiny, without even a shop. Sometimes we would instead have our main meal at lunchtime; there are some good restaurants that cater to the canal traffic. I happened to have a copy of the Michelin Guide on a smartphone and it was sometimes useful, as is a "Gites de France" app. We used a variety of sources to find accommodation. There is a Miam Miam guide for the canal, although ours was rather out of date. I also trawled the internet. We always booked a day ahead.
In Beziers and Carcasonne you have a fair walk from the canal to the interesting part of town - the canal goes through the commercial part of Carcasonne but of course we wanted to see the castle which is in the old quarter, a fair distance away. Absolutely a highlight - do allow yourself plenty of time in both towns (we took rest days in both). Investigate chambre d'hĂ´tes in both places - I think they were much nicer and better value than the hotels.
Hope this is of some use.