"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, 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."
This quote is from a great article I saved back in 2014 - published by Podium Runner. The link to the full article is here:
As you can see, the Aerobic Engine is generally the limiting factor. Easy runs, endurance runs and tempo/threshold efforts should be your 'bread & butter', with a little speedwork 'sprinkled' in.