Running on Empty

I had no idea running was so hard. I mean, I've done it, in my twenties, when it was 100 degrees, when the broadcast naysayers were like "Weather Advisory! Don't go outside or you'll dehydrate and die." So me and my other future raisin people trotted along, pink-faced, sweat-seething, amazonian warriors, and we lived. Lived and thrived to wake another day and run some more. Run s'mores. S'mores are not runny.
Meanwhile, back on this planet, ten years later, I'm out in my loosey goosey velour running pants, and my legs are so heavy, I feel like the mob half-way cemented me down there with the fishies.
(Pan to Film Noir) "Well, Delaney. Times up. You're not going anywhere but a swim with the fishies."
When I walk, my legs aren't this heavy. Could someone please properly explain gravity to me?