I once coached a college runner who ran a 19-minute 5K who said she wanted to be trained like a 17:30 5K runner. So I told her to run a 17:30 5K and then I’ll train her like a 17:30 5K runner.
It seems logical that if you want to run faster, you should practice running at that faster pace. But there are a few problems with this way of thinking:
1) What determines goal pace? A runner’s goals are often arbitrary and not realistic. I’ve coached many runners over the years who had unrealistic goals. If I had prescribed them workouts at their goal paces, those workouts would have been way over their heads, and they would have run themselves into the ground trying to accomplish them.
2) Running at goal pace, even if that goal is realistic, represents a future level of fitness. Doing workouts now at that future fitness level means that you’re doing workouts faster than what you need to run to meet the desired purpose. If that 19-minute 5K runner did her threshold workouts based on a 17:30 5K, her workouts would no longer have been purely aerobic; they would have become anaerobic, which would have changed the type of workout stress.
3) Running at goal pace moves you away from targeting the specific physiological factors that dictate running performance. You’ll no longer be training at lactate threshold pace to train lactate threshold, or VO₂max pace to train VO₂max. It’s better to target the specific physiological factors than to train at arbitrary paces.
This doesn’t mean you should never train at goal pace, but do it sparingly, only for psychological reasons to give you confidence, and only when your race goal is realistic. You’ll be a better runner if you understand and execute intensity optimization.