Because you end up wheezing really badly after the first 2k, and you don’t stop having problems breathing even if you walk the last 3.
Proof ladies and gentlemen, that I am not superhuman and should listen to my husband and NOT try to prove otherwise.
At least my next 5k time cannot be much worse. It’s hard to do worse than 40 minutes.
That said, I still beat two of my students and several of their parents.