It has been shown that the better runners mentally associate, meaning that they are focusing on their body, their breathing, their form, how they feel. This includes associating with the pain and effort of running fast and hard.
The less than better runners disassociate, meaning they tend to try to distract themselves from the effort and pain. They may look around, think of other things, listen to music, etc.
If you run any race near your maximum the last third will take a lot of effort and focus. You need to spend over 50% of your effort maintaining your pace over the last third.
Does anyone remember highly successful Indiana swim coach Doc Counsilman, who wrote about Hurt-Pain-Agony in his The Science of Swimming book? Mediocre swimmers could push themselves till their hurt, Very good swimmers could push themselves to pain. The very best swimmers push themselves to agony.
Multiple world record holder Henry Rono spoke of running interval workouts primarily to train the mind. Training the mind to accept and embrace pain and agony?
But the very best 5K runners, running their fastest times or winning the biggest races, rarely appear to be in agony during those races. Maybe hurt and pain in the last one third of a race, but not agony. They never talk about being in agony when asked about those races. They never appear to be in pain or agony when they finish. Maybe they peak properly so it all comes together on race day, and their race is relatively easy. Maybe they do the training so that the race is easier by comparison.
My very best 5ks, when I set PRs and/or won races, were done with hurt and pain during the last mile. Some of the many 5km races where I was slower than my pr, and was not close to winning, involved more hurt and pain setting in by 2k. Perhaps my inability to go into agony explains why I was never really great.
So to run your best 5k, there will be discomfort at one mile, and pain at two miles.
You want to train and prepare properly so that every once in a while you have a great 5k that seems relatively easy.