I just reviewed recent test/baseline results with an Ironman athlete. He improved his 5k time from 23:23 to 21:49 (a 6.7% improvement). This performance predicts a half-marathon improvement of 33 seconds per mile. I have no doubt he'll get it.
We’ve worked together just short of a year.
It takes time to:
improve pace, recover and go to the next level.
increase endurance without losing too much speed.
work on nutrition strategies that fit a lifestyle and racing.
have the self-control to go easy so that when it’s time to go fast - you can.
All the same training principles I used with this fellow are in my training plans.