Agree with all and especially Omar's last post. I decided to 'train' by doing easy French bit (two weeks or so) before hitting mountains. All went rather well except got shin splint three days into the REALLY up and down of Spain and had to rest a day, then ok. For certain wear the pack if you are training. Also, it isn't that you can walk a 3 or 4 hours ok, it is walking hours every day in all climates and terrain day after day. I think camino veterans will agree that it is constant the day after day that will injure you so if your body, because of training, is flexi and limber the chances are less.