Any good rucksack will do, but I'd get something you might use stalking at some other point: I recommend these Savotta rucks from Finland, which will last *the rest of your life*, and can take abuse. As others have said, get something with a waist-belt to shift the weight off shoulders into your hips. A chest-strap helps too.
Keep weight *close to your back*. 4mph (with a 5 min break at the top of each hour) is the army force-march pace, and will really help you on the hills (this was my training before Afghanistan!).
Someone above said "don't take too much weight it'll hurt you", but what is "too much"? 20kg and a fast pace will build your leg muscles and cardio vascular fitness a lot, and won't hurt your knees. 30kg? Yeah, maybe a little more wear & tear involved there. I would put your increments up by 2kg. I used a couple of weighlifting plates from Decathlon right up against the back to add manageable weight, then made the rest up with tins of beans and, of course, water!
The other component to consider is your boots... which I am sure you have. There is where you'll incur injury unless you invest in quality. The walking poles also a good suggestion.