There are as many opinions as forum members, but:
1. I'd work up to the distance first, then start adding weight very gradually. this is because doing both at once, or intermixing, might confuse you when you have a bad day what the problem was. Also, you already have known back issues. So first build the stamina for the distance, being sure shoes are right, clothes are right, etc. towards the end walk with your empty pack.
2. while you are working on building distance, do core exercises--this will help protect your back once you start adding a pack and then weight. once you can do core exercises without problem, add additional back exercises--after discussing with your doctor. If you have serious back problems, consider supervised training with a physical therapist.
3. once you have good core stability, and can easily manage the distance without weight, gradually add weight. discuss with your doctor how much weight you can carry for how many days without aggravating your back problems. consider using a pack transport service if necessary. the goal is to reach SdC, not return early for back surgery.
just one opinion...
buen camino