I was an obese kid starting at 6 years old. I remember just how hard it was to exercise in such a condition growing up. One thing I remember most was how much it hurt to breathe while exercising because I was so sedentary. Eventually in adulthood, I started exercising regularly and that breathing pain went away. Later on I found out that this is called VO2 Max training - increasing your lung capacity. Note that VO2 Max is exercise specific. The better your VO2 Max is, the better you are performing at a given exercise and ultimately, the fitter you will be.

http://www.livestrong.com/article/447500-vo2-max-training/