I've always wondered what happens psychologically and physically when you hit that state where you can pretty much go forever. It's like I begin daydreaming an

I've always wondered what happens psychologically and physically when you hit that state where you can pretty much go forever. It's like I begin daydreaming and I forget that I feel like I'm about to die from the running. One thing I noticed was that it never happened before the 1st mile, it was always after mile 2 that I would mentally drift off.