My last 5km parkrun in 20:33.
I've been through 3 cycles to date with the following 5km times.
Cycle 1 Week 3 - 20:52.
Cycle 2 Week 3 - 20:08.
Cycle 3 Week 3 - 20:33.
In all 3 cycles on the pace runs and park run's my speed endurance goes haywire at the 3km mark. I'm on pace and then slow down perceptibly for the remainder of the 5k distance.
I went sub 20 on numerous times in 2017 with my own schedule and the weekly hilly long run was I think a key workout with a weekly tempo session and a 5 x 5 minute fartlek type of session.
The difference is that I dont feel as holistically revitalized as with your program structure in comparison to my ówn previous run as you feel, weekly racing / gung ho method.
However, with my previous schedule in 2017 my wheels never came off either in the last 2k's thereforesome tweaking is currently needed.
For week 1 I have only been doing 3 x 1 mile - maybe I should increase this?
For week 2 I have only been doing 4 x 1km - maybe I should increase this?
As for introducing a weekly long run this wont seem to fit into your structure without disturbing the quality days and subsequent rest.
I have also focused on dropping my weight down from 87.5 kg to 85kg.
Height 178cm - age 52.
By week 3 I am raring to go, a bundle of energy but the wheels always seem to come off in the last 2 km even though I am pacing myself correctly from the word GO.