We evolved as hunter/gatherers. I don't think the wooly mammoth stood around waiting to be attacked by hairless animals with pointy sticks. American Indians would chase a deer til it collapsed from exhaustion and could be killed. The deer would race away from the Indians til he felt safe, but when the Indians got close he'd race away again. Eventually he ran out of glycogen and couldn't run anymore. Our physiology works best with fat-burning for long, slow distance with brief spurts of all-out effort. That's why we have mainly fat stores but very little glycogen stores.

BTW, I'm 50 and my joints are fine, thanks.