But what if by walking faster you improve the common factor? What if it's just that gait speed is a symptom of poor heath, but by attempting to walk faster (maybe even by going for more walks) you actually improve your health?
It's pretty common-sense that healthy people will already have a faster gait. I mean for lots of people who are about to die, their gait speed is probably zero because you can't even walk. Your average person doesn't even spend much time exercising, and this study is about average folks, so it's saying that average folks with a faster gait live longer. Most of them don't even exercise.