Margaret's advice is spot on! I walked some before my camino, but never with my pack. Training with the exact equipment you plan to use on the camino will give you an opportunity to see if it works for you and what adjustments need to be made. I believe it is possible to gradually get in camino shape after you start the camino, however, if your equipment (socks, shoes, pack, etc) are issues, it could be a painful learning curve. What is recommended by someone on this forum may not work for you. It took me more than a week to figure out the best adjustment for my pack, even longer than that to get the best sock setup for me, and unfortunately too long before I realized that I had to dump about 30% of the weight in my pack. The consistent advice on this forum that should be heeded is to keep your pack weight less than 10% of your body weight. The happiest pilgrims were the ones with the lightest backpacks. You may feel as I did that this doesn't apply to you, and that is where walking for long distances with your loaded pack before you get to the camino will reinforce this advice.
Buen Camino,
Rickster