Hello Philip,
I have walked the Camino from Saint Jean Pied de Port to Santiago in September 2006. Yes, it is very realistic to walk all the ways to Santiago in 31-32 days. Walking 20km per day at the moment is more than adequate training. You can get the credencial in Roncesvalles in the pilgrim office which is across the road from the Roncesvalles Albergue. I dont have any information about a taxi from Pamplona, each time I flew into Biarritz. As regards the weather, I had 28 days of sunshine, no need for a goretex jacket, you will not need it. Just carry a fleece and a poncho, if it does rain, the poncho is perfect as it will cover you and your backpack but it will also allow air to circulate, and the poncho is much lighter than a goretex jacket. Compeed is available in all the pharmacies along the camino, as well as any other pharmacy items that you may require along the way.
Have a great Camino.
Paul.