My thoughts on this. You know, a lot of people talk about training on here, but few do it. And honestly, if you're in fairly good shape, I'm not sure it's necessary...
GOOD SHOES have been mentioned and I think more people get hurt from having ill-fitting shoes than any lack of training.
The number two injury causer in my estimation is a too-heavy pack. Carry the suggested 10% of your body weight and you'll be fine. There is nothing you need that you cannot buy along the way, if you have shoes, backpack, and a change of clothes.
Again, I suggest New Balance walkers or trail shoes with the WIDE TOE BOX. Some people love their boots, but I saw many pairs of boots discarded along the Way. They're overkill for many, unless you have really weak ankles. They also do not dry as fast as walkers, and the thought of putting my feet into cold, stiff, wet boots doesn't make me smile.
Your toes need to be able to spread out too, so be sure the shoes have room in the toe. Buy the shoes one size too large, as your feet WILL swell. You are not used to walking straight for 6 hours per day, and your feet will complain if you don't take care of them. Wear the suggest liner sock with SmartWool over the top. On hot days, you can just use the liners.
Mermaid Lilli wore Teva sandals and loved them. I tried wearing my Tevas but they gave me blisters, so I saved those for after hours in the alburgue.
Stop and take care of your feet at the first sign of discomfort. That means carry compeed with you. Buy it at your first stop and keep it handy. Slap it on any hot spots BEFORE they become blisters. Keep it on until it falls off on its own.
There are few places on the camino that are difficult. The first is hell-day from SJPP to Roncevalles... if you get through that, you're home free. Just go slow, walk at your own pace, don't rush for a bed. Many people walk the entire stage. I suggest staying at one of the in-between places the first night, even though it only cuts off a few kilometers, it will make a HUGE difference and can be the difference between heaven and hell, in my opinion. I walked the entire stage at the suggestion of the hospitalero in SJPP and I cried every step of the last 2 kilometers. I should have followed my first plan, and stayed the night 5 k into the walk.
Once you pass Roncevalles, the rest is easy walking and you get stronger by the day... after a few days, you find your stride, and you basically train as you go.
We took some 2 mile and 4 mile hikes for about a month before we left, but I wouldn't call it training... mostly to check our shoes for fit and our backpacks.