I have a horseshoer's back, and carrying a heavy pack doesn't really bother it at all. I also work out quite a bit, and do a lot of core work, but I've found that the best way to prepare your body for carrying a lot of weight, is to put a pack on and carry a lot of weight. I try to make at least one of my workouts every week hiking with weight, or going to my kid's school, and walking up and down the bleachers with weight on my back (added benefit of making my kids cringe). I think as long as your back is healthy enough that you aren't risking further physical damage, and you build yourself up to be able to carry the load you expect to carry you'll be good. Most of the big name pack makers make good products, find the one that fits you the best, get some good trekking poles, and give it a go!