I really appreciate your question. It is more subtle than it seems. I coach largely following a Lydiard-informed methodology. This sounds nice but really it just means emphasizing the aerobic condition, sequential development, and running time-based.
Here is a sample week during base building:
Monday, off
Tuesday, 90mins 1/4 effort w/ 6 strtides
Wednesday, 45min recovery fartlek
Thursday, 90mins 1/4 effort w/ 6 strides
Friday, 45min jog
Saturday, Out & Back Run | 31mins Out 1/4 effort + 29mins Back 1/2 effort
Sunday, 2hrs 1/4 effort w/ 8 strides
If you are respecting this polarized, time-based model, what is going to happen over 10-12 weeks? Your aerobic condition will improve. As a result, so will your moving speeds. Let's say week 1 of the Aerobic Conditioning Phase, you follow this sample week to the letter and come out with 50 mpw. After 4 weeks, you should be seeing that you are logging 50.5 miles, or 51 miles, or 54 miles (it depends on your history, talent, and how diligent you are at hitting the correct training intensities over weeks and months), yet your heart rate, or felt-sense of vitality, has remained static. Does this make sense? Weekly duration should remain the same while mileage increases. If you are not seeing your mileage increase as a natural byproduct of aerobic training, there is an underlying health issue or its simply time for you to nudge the pace up manually. Aerobic advancement does not happen automatically. You want to be building a clear channel of communication between you and your body. You want to ask yourself if, after your typical Tuesday medium-long run, you will be fully recovered by Thursday's medium-long run. If you are very well recovered, then it is time to ask yourself what it would feel like to quicken the pace very slightly, and generate an additional recovery cost but one that is still absorbable so as to never need down weeks (unless for illness or low excitement level).