Ive only done bits of it (and there's about 4 different routes). The bits ive done are in the Baasque country it went past my old house) and there's a mix of road, easy tracks and proper off road stuff (but usually only short sections). You can always push. (some of the hills are pretty damn steep).
St Jean Pied is a good place to start, it's a nice enough place that specialises in Basque tat. The Roncavalles pass is very nice and the trail coming down the Spanish side looks excellent for riding.
bear in mind, this year is a special year for the camino and is going to be very very busy. There's some lovely places along the route though, I'm sure you'll have a good time.
