I just realized that just before it stopped running, it ran really really
fast. Of course, I had protective headgear on, but, even then, thinking
back, it was running really fast.

I wonder ...

I wonder if I blew the rings somehow.

When I put my finger over the hole after removing the spark plug, and
pulled the starter cord, there was only a small puff. Noticeable. But
small.

Do you think compression might be the problem?