Hey lévrier, your 5k run objective.
I am convinced you already have the basic speed to run 1km at that pace - possibly longer.
And then im also convinced, with enuff rest in between.
You could run 5 X 1k @ that pace - possibly faster.
So the reason why you cannot MAINTAIN that pace for 5k.
Is because you lack the ENDURANCE and STRENGTH.
Especially when theres a swim/bike in front of that 5k.
The key, is to run bellow, at and above that pace
Some short, some medium and some long.
How much, how often and how long?
That depends on particular individual factors
Of which a competent coach will narrow down to and prescribe accordingly...