To create a training program that helps your running progress, you have to pay close attention to what it is that particularly limits you… Take for example the runners who decide they're held back mostly by their breathing. If they take the cautious approach – 'Oh, I should NEVER get out of breath' – they won't get much of a training effect on their lungs, which is precisely what they need most. They won't learn to cope with being out of breath, nor will they learn how to recover after having been out of breath, which are abilities they'll need if they're going to race well. A runner who trains without getting out of breath won't give his heart much of a workout either. And if his breathing is the first thing that tells him he's working too hard, his legs won't be getting stronger, because they won't be put under any pressure. The result will be that nothing much changes. Next year and the year after, those limiting factors will be just as limiting. And that runner will still be posting the same times he does today.