LukasGabriel (sorry, not sure which you would prefer it shortened to), that would be a big touché on that first one. I honestly don't know what I would do if I had a dog like yours -- I keep forgetting how fortunate I am with my current dog in regard to health issues (even cancer, given she'll be 7 next year and is part Boxer) and all that jazz. I live "in town" and my favorite hiking spot (dog's too) is just along the river; admittedly part of that is because it's close, but it's a nice hiking spot with enough variation to keep it interesting and still get a good hike out of it -- plus she can cool down by walking in the water (or falling in on occasion).
Swimming definitely is an awesome work-out for the entire body, and easy on the joints as well. Didn't think of it either because I only just recently learned I can, in fact, swim...somewhat at least. Another great full-body work-out, actually, is all-out 4-wheeling -- a day of that and even toes and fingers have been worked out, haha! But, given you're in the city, suppose that would work even less for you, Alex.
Curious, Alex, does your city have a park with walking trails? Maybe you could work with that?
I had found in the past that crunches and sit-ups have more of an effect on an incline bench (those are usually padded) than on a flat surface like the floor or something. But, for the best effect there's other exercises, and ones that are easier on the lower-back as well. I did find the incline bench alleviated some on the back for me, but not a great deal.