Physical attributes are not particularly important wrt promotions in the military; you just need to pass that baseline fitness bar to not be flunked out on medical.

Indeed, the higher up you go, the more your job becomes about leading and the less it becomes about individual feats of strength. I think you're thinking about the military a little bit wrong here -- this isn't the Roman era (when centurions led from the front and had to be legitimate physical badasses). Being higher up in the military nowadays is a lot more akin to being a paper pusher director at a company.