Staying fit with health problems?

I have two major issues that prevent me to do much workout: weak lungs and back pain. My back is usually fine unless I do repetitive movements or stay bent over for a while, and my lungs, well, they just can't get enough air when I'm not even doing anything, so working out would be terrible.

What kind of activities (besides sex, which I'm fine losing my breath for) could I do to stay in shape? I'm mostly worried about my belly and seemingly developing muffin top.
Staying fit with health problems?
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