I remember having to do the mile run in elementary school every year as part of the Presidential Fitness Challenge. Mile run, pull ups, sit ups, sit & reach. I never did any good on it. I always encountered this problem of my lungs not being able to keep up with me. I'd run an eighth of a mile and my lungs would start to burn and get tight. I was never tested for asthma. Chances are that I was just really out of shape and my lungs (like any other muscle) just weren't fit.
I remember as I got older that I just stopped trying. I knew I could walk the mile in 15 minutes and that was good enough for me. Some people would run 12 minute miles, and I was just 3 minutes behind while putting out far less effort. In other words, I gave up and settled with mediocrity.
By the time I started going to the gym, I was convinced that my body was holding me back. I must have had asthma. I was convinced I was fat because I had PCOS. As it turns out, my lungs were just underused and weak, but I could exercise and condition them. And I didn't have PCOS. My excuses couldn't excuse me anymore.
So I went to the gym. I let someone else expect more from me than I did of myself. Lemme tell you, that first kilometer on the elliptical was absolute murder and I thought I was going to die. But in a few weeks, I was able to work myself up to a 10k in 52 minutes. The big surprise was that my lungs did not fail me -- they actually liked the work out. As I would get going, my lungs would relax and take in more air. The fatigue in my muscles would go away because my body was dealing with the lactic acid build up. I was compensating!
Certain exercises were still hard for me (as I'd later find out, my back injury was more serious than just not having strong core muscles, but having bulging discs that affected the nerves in my back).
But I learned that the most important part of changing your life is to expect more from yourself than you thought you were capable of. And if you're not up to the challenge, fake it. If you don't have the strength, borrow some. The bottom line is that you'll never know just how strong you are until you push yourself to your limit and then some.
I don't believe in the phrase "No pain, no gain" (because pain sometimes tells you something is wrong) but I understand what it's getting at -- there's a difference between mental pain and actual physical pain. Push past the mental construct of what you think your limits are.