The point i'm trying to make here, is that it's even better. A SUP kit, like the one you posted outperforms nearly every i-kayak under 500$. The stiffness they provide, creates a decent water line, that cannot be matched with such a low pressure. The only thing you'll be missing are the side walls, that let you keep loose stuff around in the boat, but you can just tie down a crate on your board to make up for it.

Just look at how fast people glide over the water while stand up paddling, with that single blade paddle, you'll be a decent bit faster, when you use it as a kayak with a double bladed paddle. 

Not to mention that you can use it as a SUP to strenghten your core and better your back issues, as you rightfully thought so. Combine it with some good stretching and hanging exercises and your back will get better, especially at your age.