It's my understanding from my reading of pop-science books (specifically, Nick Lanes) that mitochondria can be selected for within your own body. As you age, m

It's my understanding from my reading of pop-science books (specifically, Nick Lanes) that mitochondria can be selected for within your own body. As you age, mutations naturally produce slightly broken mitochondria who do a slightly worse job at making energy. However, if you are exercising vigorously, those poorly functioning mitochondria will die off (apoptosis) and you will maintain a (relatively) "fitter" population of mitochondria inside your muscles, as those are the ones that are left to reproduce. If you don't ever exercise, you'll probably feel fine (all else being equal) but secretly, you're cultivating a weak population of mitochondria within your own muscle cells. At some point the weakness of that population will become apparent.