You are not so unfit as you say. I'm at least 150 lbs overweight and cannot even lift my feet from the ground, and have spittle that congeals to a solid mass in my lungs with the very slightest of exertion. YOU may have been designed to run, but I am not. I am more designed for handstands than I am running, and I can't even do handstands because I'm too fat. And yes, I've tried running several thousand times, it's excruciatingly painful and is causing me incredible pain just remembering it. They say trying to do something over and over again, expecting different results, is a sign of insanity. Or science. The line between the two isn't very clear.