The true limiting factor on how far and fast I can walk or run has rarely been my legs, after about age ten. It's been my lungs.

I remember the day it started. I had been sick, but I'd gotten better, and it was one of those summer days where swallowtayle and the Suzuki-method violin students had been playing music at the Museum up on campus. We ran around like maniacs, and I was running back to the car when I felt my breathing suddenly get worse. My lungs were clogged up again, and I was sick for another week or two.

I was more careful after that.

It happened again in the freshman year of high school. The gym teacher had us running sprints, and we'd done something or other wrong, and we just kept running and running...

I got dizzy and my lungs did that nasty thing again. Goodbye, health for winter break!

I'm plenty strong. If I do things that don't strain my lungs, I have decent endurance. I just can't do anything that makes for heavy breathing when I'm sick, because that makes it worse.