Barcelona, Spain to La Rochelle, France (by way of Perpignan?)

We are planning on cycling on a tandem from Barcelona to Atlantic coast of France. I've been doing research, I see that in France there should be nice bike trails "pistes cyclables" along the "Canal du Midi" and along the Garonne. However, we are uncertain what easiest route over the Pyrenees would be. We would like to find a route with few cars, some options for stopping overnight, and preferably not too difficult, since we are not young and super-fit!
Any ideas? Seems like there may be a couple of options heading towards Perpignan, though it would be interesting to visit Andorra ... :D Probably not though...