Something strange happened to me a few years ago. I was never aerobically fit as I was never able to run more than about 2 blocks (at jogging speed) without becoming short-of-breath and having burning sensations in my leg muscles. Then, on one particular day, during a warm-up run for a high-school PE class, I was able to run about 20 blocks (switching between moderate and maximum speed) without becoming short-of-breath or having the burning sensations in my legs. I was completely taken by surprise-so much so that I actually wanted to run the entire circuit again when I was finished. Throughout the rest of that PE class, I also noted that I was able to run the entire length of a soccer field without becoming tired whatsoever. In the past, I've always noticed that I was unable to keep up with the class in sports that involved a lot of running but on this day I was able to. In summary, in all the days leading up to this day, I was only able to jog for about 2 blocks before becoming short-of-breath and tired, etc., then on this particular day, I was able to run at fairly high-speed through 20 blocks without experiencing these symptoms, and in the years since that day, I've been at my original poor fitness level. Would someone please explain how this could have occurred? I've been pondering this question from time-to-time in the 8 years since while at the same time, trying to improve my aerobic fitness level. (By the way, I am not overweight.)