I should have added a few extra bits of information.
First, there is almost certainly a route which crosses the Pyrenees at the Western end - and there will certainly be high routes through the Pyrenees. Our route was deliberately chosen to be as flat as possible.
Second, from the Spanish border there are perhaps 20 miles more of tortuous coastal roads before you can strike inland on the plains down to Barcelona. I'd guess it's another 100 miles or so to get there. If we hadn't had to get back to work I think we would have been very tempted to keep going!
Third, there's a very useful website -
www.canaletvoieverte.com - which includes the latest maps of the canals and bike routes I mentioned - two cover the whole East to West route. They include details of campsites and B&Bs. Paper copies are theoretically available when you arrive, but distribution seems to be poor. The section of the Canal lateral du Garonne we diverted to Montauban to avoid was the only section that our main guidebook suggested was unsurfaced, but signs on the canal suggest this was rectified this year and last.