Random Question - What is a second wind? Anyone?!

Your body has different energy sources. One source uses the glycogen in your blood and liver to produce energy.  Your body can generate about 2,000 to 3,000 calories from the glycogen you store in your body.  When you hear of people carb loading before a race there are building up their storage of glycogen.

In a race you are probably going to burn about 1,000 calories an hour.  If your race is more than 2-3 hours long you are going to exhaust your glycogen storage.  When you get to that point you hit what is called the runner's wall.  The first time you hit that wall your body will scream to stopping in your tracks and not taking another step.  That is when your first wind is gone. 

You have to push through the wall.  If you stop your body won't need to continue to generate energy and you stay in that zone where you are out of energy and feeling dead.  If you keep going your body will have to  switch from burring carbs to generate energy to burning fat to generate energy.  It takes about 10 minutes from the time you hit the wall to where your body is burning fat.  As the fat burning cycle take over you come out of the dark zone called the wall and you start to feel good again.   Feeling good again is what is called your second wind. 

Fat doesn't burn as fast as glycogen does so you won't be sprinting when you come through the wall.  You still need to pace yourself and take it slow but you won't feel like you want to stop anymore.  You actually will feel pretty good.  The chemical processes to break down fat are said to even give you a runners high which is why some people say endurance sports are addicting. 

The more times you go through the runner's wall the easier it gets to transition.  Your first time running off the bike you legs feel like jello and you have a hard times walking straight let alone running.  After doing you brick training your body gets better at the transition and you can run off the bike like it is nothing.  The wall is the same way.  As you learn to pace, refuel, push through the wall etc.  You just feel a little sluggish for 10 minutes rather than feeling like you are going to die.  I personally don't know if I have ever got a runners high.  I defiantly have learned to feel when the wall is coming on though and when I have caught my second wind.

P.S.  What you are experiencing in a work out is not likely to be moving from the Krebs cycling in to ketosis.  When you are working horses they get fatigues and slow down until you turn to back to the barn.  When they know they are almost done they not longer have to pace them self for infinite life and can put all that they have left into a short energy bust.  They also say that no matter how dehydrated people are that get lost in the desert that when they rescue team finds them that they can still cry tears of joy.  Your body saves a little back.  When it feels it can use it it will release the emergency supply.  When I get the end of the work out burst I know the horses are headed back to the barn.