Hi
I'm probably too late but, since I live just 20km north of the canal, I thought I'd join in anyway. The path is surfaced from its western end until a little after Carcassonne (see below for a better way out of Bordeaux). There are more and more root ruts before you get to Agen, sometimes several in a row, but it's still ridable and better than the alternative. Just before Agen is the bike café, where there's a workshop, a restaurant and accommodation for 12. The path vanishes going through Agen to become a quiet canal-side road; ignore the signs that suggest you don't go beneath the bridges.
Just before Valence d'Agen, 20km further, is the old abattoir that is now a fairly basic cycling stop. There's a campground in the town but you could also camp here.
In Moissac, the path becomes a canalside road. Look for small direction signs as you get close to the town so that you fork to the right instead of going into town. Then look for signs for the canal and the river, which are close to each other but go to different places. You want the path to the left.
All will be well then until after Carcassonne, where the surface ends and the path becomes really a hiking route. Later on it is simply unridable unless you like that sort of thing. I'm less familiar with it at the Mediterranean end but I know that at some stage the good surface appears again.
If you're riding from Bordeaux, the Lapébie path is a better choice. From the city centre, cross the river to the north and the path will start immediately, on your right, first of all as a segregated bike lane. It will soon turn into a beautifully surfaced path with no tree roots. When you get towards the end, turn right and south to pick up the canal.
I hope that helps.
happy days
léo