Let's compare two runners who want to PR in the 5K.
Beginner Brad has been running for two months. He read the PR in the 5K article for new runners, and started doing strides, boosted his long run, and completed a more structured workout each week. Within six weeks, he cut over a minute off his PR!
Advanced Allison has been running for five years. She already does strides, workouts and long runs. But she wants to lower her 5K PR.
What is Allison to do?
Runners who already follow a more structured training plan are going to be closer to their potential than those who just run a few easy runs every week.
To improve, more advanced runners need to adjust their training and take the next logical step. Note that "advanced" does not mean fast—it simply means your training is structured and you've been running for about three or more years.
PR Principle #1: More Mileage
As my college coach would respond to the question, "How do I get faster?" The answer is mileage.
The simple truth is that most runners aren't running enough and aren't realizing their potential because of a lack of endurance. The 5K is primarily an endurance event so you need to train accordingly.