I’m not Stilljd, but, i read it as:-
If she keeps improving in every 2nd or 3rd race, eventually, after 4-5yrs of doing so - she will finish before the Gun even goes off.
What he is saying - PB come and go - if all you are chasing is PB - one day they will run out, and the only way to run a PB is beat the GUN…
Or reduce the amount of PB’s being achieved, which means, if you only LOVE pb’s, you’ll no longer Love the sport.
That is how i read it 