Yeah, I rode the left bank of the Gironde to Pauillac then cut across on the paths to Arcachon, then stuck to the coast mainly, down to the border (I was heading North-South, obv). The paths are nice - quiet, cutting through the pine forests largely away from the roads. If you're planning to stop at a particular town/campsite keep your eyes peeled - sometimes there are no signs telling you it's time to get off the cycle path!
Mt Jaizkibel tops out at 455 metres and you start it from pretty much sea level. It's pretty straightforward with some great views and a nice break from the main road! You can see it quite clearly on Google Maps (or similar) heading West from the town of Hondarriba, along the spine of the little peninsular thing.
If you're planning to camp in/around San Sebastian be aware that the nearest campsite to town is on the small coastal road into town (the way you'll be coming at it) near the small village of Igeldo. It's right up on the cliffs and a good 300-400 metre climb from town - not somethng you want to discover having just descended into the centre! And not the best place for a fleeting visit to San Sebastian either, especially if you want a rest from the bike (although I guess there probably are buses).