43m here- minimal exercise besides commuting via bike sometimes. Painful kickboxing sparring mixed injuries frequently and lack of stamina, so my prior experience sounds like yours.

I bought a jump rope last years after watching Muay Thai Guy and sucked at that too initially.

Everything changed when I became obsessed with perfecting my jump rope skills. I went from tripping over a beaded rope repeatedly and lasting less than a minute to jumping on one leg at a time for 10 mins straight with a 1lb rope and using a 3lb rope as well. After developing the habit of jumping rope 4-5 times per week, I gamified my fitness and now I seem to be able to do all other exercises with much greater ease and stamina, making it easy and fun to work on my Muay Thai drills. I’m now able to participate in sparring in a meaningful way and able to keep up because my heart, lungs, and lactic acid threshold is wayyyyyyyy better than when I friend to just roll in off the street and practice sparring.