One hot, humid Sunday morning last August, in the middle of marathon training, I set out for a 14-mile run down the long stretch of road from Atlantic Beach to Pine Knoll Shores to Indian Beach and back. No, you can’t see the water. About a mile into my run, I noticed I was running significantly faster than my usual pace. Can I keep this up the whole time? And then, this thought popped in my head—IF I …