How should I start my training so it's not too hard on my body?

No matter what you do, it will be hard on your body, but there are things you can do to make yourself more resilient. First off, I would start a phased workout plan that involves different focuses over time. So I would start with running then go to swimming, then Cals, then rucking, and so on. Increase the weight you are able to carry and make your body used to it in order to reduce injury chances later on.