Went from Roscoff to Bayonne last year and included a fair bit of Velodyssey route.
It varies. From Roscoff it's back roads to Morlaix, then old railway lines and canal path to around Nantes. Generally fine grit, some tarmac. Slowish surfaces but blessedly traffic free. From Nantes I generally used side roads until the last 100 or so miles, which was almost new tarmac paths through the forest. The last bit from Cap Breton to Bayonne was roads and I might have missed signs in the rain, but didn't seem well signed. Plenty of camp sites and B&Bs, but probably best out of the main French holiday from around mid July to end August. The canal paths can get a bit boring after a while and there were a few rough sections following the flooding.