Don't believe me? Try this the next time you go for a walk around your neighborhood.
Don't think about anything but your breathing while you walk and walk at a slow relaxed pace. Take a long, slow, deep breath in through your nose. Make the breath as deep as you can make it and then hold it for two steps.
Now let the breath out slowly through your mouth but make a ssssss sound through your teeth. As you let the breath out tighten your core muscles as tightly as you can. One you have forced as much air as you can out take two steps before you repeat the process.
How did it feel? I bet you don't make it very far breathing like this.
So how does knowing that breathing is exercise too help you? You have to make effective use of breathing during your workouts to maximize all the energy you have stored up in your muscles. If you aren't breathing effectively you will fatigue more quickly and it will take your body longer to recover between workouts.
Later I'll make a post on how you can use breathing to strengthen your core and increase your energy every day.