How much speed do you need?
From a training standpoint, speed is rarely the limiting factor in how fast you can race, even for a distance as “short” as the 5K. Let’s look at this idea in more depth as the great coach Arthur Lydiard once did when he popularized training 800 and 1500 meter runners with a steady diet of 100 mile weeks.
If you want to run 20 minutes for the 5K, you need to average 6:25 pace per mile. Technically, that means the fastest pace you need to be able to run is 6:20 per mile. If you’re a 21-minute 5k runner or a 3:25 marathoner (roughly the equivalent of a 21-minute 5K runner), I have little doubt you can run a 6:20 mile; you’re probably capable of running a mile close to 6 minutes. Thus, the problem isn’t that you don’t have enough speed to run a 20-minute 5K, it’s that you lack the endurance to run three 6:25 miles without stopping.
Therefore, when you’re examining your training and identifying your strengths and weaknesses, the most obvious areas of improvement are going to come from improving your aerobic endurance and lactate threshold. If you can already run far faster than your goal race pace, the problem isn’t your speed.
Aerobic capacity and lactate threshold, what running coaches refer to as “strength” work, are the backbone of your ability to sustain a fast pace for a long period of time. In short, the higher your aerobic capacity, the longer you can run near your maximum speed.