Something’s definitely wrong

I barely got a hundred yards before my legs started aching and I was exhausted. Im definitely weaker for some reason. I was getting to running half the journey without stopping and now suddenly I can barely run at all. Ive always had low stamina as I can barely run a mile despite years of training. I don’t understand how people can run miles daily, I exorcise my ass off but never get any better. If anything Im weaker than I was years ago.