Hi Z, whatever the explanation of American slowing may be, your own transformation through the exertion of long distance running resonates and makes us all think about what we are doing in our lives and why. And that is very important.
The search engine shows that this study has received wide news coverage and some pushback. The pushback mainly involves offering morally flattering explanations, such as more less-fit people are running and that is morally good. Here is one from npr:
http://www.npr.org/sections/13.7/2017/0 ... ing-slower
As for me, I run only for fun and mostly intervals, not for any moral reason. I admire the fast marathon runners including the nonprofessional runners like you Z, for the sake of beauty not goodness. I love the gracefulness of the fast runners, and wish I could be so graceful in my movements. I know hikers that are also beautiful to see in this way.
I wish Americans could increase our pace. It would be beautiful to behold.