This is what I'm trying to align to better. Even most running programs I've seen are generic and something like LED 20min this week, 25 next week, 20min tempo run this week, 25min in three weeks. Run 4x400m intervals with 90sec rest this week, run 5x400m intervals next week, then reduce the rest to 60sec in 4 weeks. Do this for the length of the program and you'll find out where you'll be at the end.
What if you're running too hard on those intervals, what if you're not running hard enough on them. What if you're ready for more than a 5min extension from week to week, what if you shouldn't jump more than 5min every couple weeks. What if 4 intervals is too many or not enough, etc., etc.
I don't want to just run and see what it results in X weeks from now or much worse, go too hard and get slower or injured. I want to start with the end goal, work backward, do the work and achieve the goal. If I want a 5k time of X, I'd like to work backward and progress at a sustainable rate and then go back however far it takes to make that progression happen. Just like landing a plane, I don't want to come in too steep or too shallow.