you need to recognize energy systems
aerobic anaerobic gas tank
different tissues take different time to repair
nutritional deficiency mineral can take months or more to bring back
aerobic adaptation is continuous moderate stress like 70 or so minutes per day
anaerobic adaptation is every other day or every three days intense repeats
gas tank is 90 plus min runs without fuel, or with cross training to adapt
taking a 3 day holiday regularly from running shores up a lot of deficits and allows repair
if you are not running a deficit, you actually don't need days off.
i think once you start setting PRs and have a great streak going, take a day to 3 day holiday, and come back with something like a 70 minute steady run and back at it.
then there is the every day adaptation, like 10x150m strides, which tunes specific firing and muscles, with minor anabolic effect.
to keep your speed you want to monitor the catabolism vs anabolic
over the years distance runners adapt to the net pace of their work, which compromises so called fast twitch, and you need hard high intensity work with full recovery to build back up, and up the protein as well, aka body builder style (moderate) to get a quarter pound muscle in exactly the right places.
this is the alchemy of training, and everybody's body is different, and different every year.
after you have a big engine, you need less weeks on aerobic volume, and can get at the threshold and speed quicker, and also fit in small holidays