PBs are possible, I think. I ran a Parkrun PB of 24:04 five months ago at Stretford. It knocked 30 seconds off my previous PB, which was about three years ago. I'm 61, and I thought PBs were consigned to the past too.
Since then though, I've had a three month injury lay off, so I'm not close to that level at the moment.
I'll qualify this by adding I used to run when in my mid twenties, and I ran much faster back then. This was when Garmin used to be a compass and map though, and although I have no records, I have a vague memory of 17 minutes for a distance since measured at roughly 5km.