Your main problem is your feet and how much training they and your boots get before you start.
You do not need to be super fit to do the Camino, but your feet and boots need much training together, and your leg muscles need stretching and getting used to constant walking. On the Camino you will do up to 25/ 30kms per day, every day for 4/6 weeks and that means that before you start the boots must be well broken in and your feet used to constant walking.
Start out with a week of doing 5kms per day and then increase the distance each week before you leave. Find a long flight of stairs (30 steps+) and go up and down for an hour per day. This will stretch the leg and thigh muscles and prepare them especially for the downhill parts of the Camino. It is easy walking uphill, the problems start when coming down!! If your legs are not used to the stretch needed for long rough decents, then you will end up with very painful knees.
After the first week on the stairs, start wearing your pack and gradually increase the weight in the pack until you get to the 10kg max you should be carrying.
This of course assumes that you have nothing better to do during the day than to plod up and down stairs. Any time spent in a gym will be worth it (as long as its not in the bar!!) but the main area you need to concentrate on is from the waist down.
Upper body strength and fitness is not so important, but getting your feet used to constant walking in boots is vital.
Covey