Not to scare you, but I just finished from SJPDP to Santiago and know that walking the camino is much like child birth. It is amazing how quickly we forget the hard days, the blisters, the legs that ache at night and the pilgrims who went home. I trained around 80k/50 miles a week but nothing prepared me for the hills. Even though the elevation map shows areas without hills, there is always a hill around the corner. That being said I would do it again in a heartbeat. I followed some very sound advice "start like an old woman and you will finish like a young one." My advice:
1) Stay at Orisson. Not only for physical reasons, but we forged many bonds that lasted throughout the entire Camino.
2) use pacer poles!!!!! Or at least TWO poles. Saw many a pilgrim end early with a stick.
3) if you walk the Napoleon route, take the alternative route down. It isn't clearly marked, but the information you receive at the pilgrim office in SJPDP has a good picture and explanation. We didn't pay attention and took the old roman road. This ends many a pilgrims walk.
4) don't walk to many km's in the beginning. You will be able to make up some time on the Meseta and at the end when your Camino legs kick in.
5) consider sending your bag over the Pyrenees, particularly if you are older or have weak knees. This will allow your legs the time to build up on this big climb without the extra weight.
6) know how to take care of blister and have the right supplies. This was the most important item(s) I carried in my pack. I read a book that I downloaded called "Fixing your feet." I helped more than a few pilgrims with my supplies. The best item to have is what we called a "toe condom." I am not sure what they are really called, but you can buy them in the farmacia. They are silicone lined tubes that you cut in length to fit your toe. They are amazing and worth the 8eruo. You can wash them and rewear. Also be sure to buy a cream that has ibuprofen or voltaren in them. These are prescription in the U.S. But can be bought over the counter in France and Spain for only a few euros.
7) I walked in compression pants and loved the support they provided they really helped with sore legs and I didn't get any tendinitis. My walking partner got shin splints and started wearing compression sleeves and she had a quick recovery. The compression pants I wore are by SKINS.
8) build in rest days (at a minimum in Pamplona, Burgos and Leon.) not only will your legs welcome them, but these are great cities worth spending some time in.
9) take breaks during the day to pamper your feet, get some fuel and smell the roses. Try not to make it a race. It really doesn't matter if you reach your destination at 1:00 or 3:00 as only laundry awaits you.......so keep your mind and body healthy.
10) shoes are a personal choose and we saw everything that worked and everything that didn't work. Many people had bought their boots to small and after a few washing and drying their socks were too small. I personally could not have imagined walking in anything but my Hoka boots. I appreciated the ankle support on the very rock paths and steep descents and appreciated the amount of cushion they provided and was glad I could barely feel the rocks. They worked beautifully FOR ME and will say that everyone I met wearing Hoka boots or trail shoes were very happy.
11) pack LIGHT. Exactly what you need depends a little on when you go. Just remember you only need one to wear and one to wash. Don't take ANYTHING extra........a heavy bag will cause your hips to hurt at night and keep you from sleeping.
11) Listen to you body and you will have the most amazing and life changing experience.