I was very thin (but not bony) as a child but had no stamina AT ALL. In 7th grade Phys. Ed, we did each type of sport for several weeks. When we did track and field, I never did gain any increase in lung capacity. After six weeks of daily running, I hurt just as bad, running anything 220 yds and over as I did the first day. Lungs burning, side stitches, the whole picture.