Second Wind:

If you ask a runner if they've ever experienced "Second Wind" (not my book - 'The' second wind) and they respond with anything like "uh" "well" "huh" or "I think this one time" then they haven't.

Second Wind is a pure high that is unmistakable - a time when you're tired, your breath is shot, you're inching closer to the dark side (quitting) and all of the sudden the heavens open up, the stars align, God smiles down upon your wandering feet and you... fly.  It's a high - you can all of the sudden run at peak performance with seemingly no effort, your feet, muscles, heart rate, and breath all of the sudden feel like you've just started your run, and 'that' undeniable grimace turns to a smile of elation.

Some people claim that this is a moment of your body's oxygen level balancing out perfectly with your muscles, some that it's a balance or 'flush' of lactic acid, some say it's a sudden and enormous release of endorphin, many others that it's purely psychological, and  still others that it's simply a myth.  What most runners will tell you who have experienced it, it's a high like no other.  They will also tell you that if you are a 3r (run 3 miles 3 times a week) you're likely to never experience it - that it usually occurs at long distance, the earliest many people ever experience it is 7-15 miles, still others say nothing other than an Ultra triggers it.  Everyone will tell you one thing.  They don't know.  We don't know when it will happen, we don't know what causes it, it happens rarely (sometimes only once - sometimes never) and it can't be replicated.

The runner's second wind is like a good orgasm... if you go out Trying to find it, you likely never will.