In my opinion, there is no definite answer.
Look throughout the history of running performance. You will see that seemingly opposite methods led to similar results e.g. Lydiard with high volume base period and then peaking with high intensity pre-competition.
Then you've got someone like Igloi who builds everything with intervals.
Credit to Science of running who made a good podcast about this question exactly.
You could understand then that
The numbers you mention regarding the split between base endurance and more intense work (intervals, ...) can be found also in the work performed by Seiler on elite endurance athlete. He states that elite endurance athletes seem to display 80% volume at base pace and 20% at high intensity. And no "junk" work at medium intensity. Keep in mind though that one speaks about elite athletes who have plenty of time to train and to develop.
Maybe if you are an average Joe that does not have 5 hours per day to train, then modulating the ratios will be necessary for you to obtain the performance you desire and/or simply enjoy your training and fitness.
Now back to your question : How do you know if what you are doing is right ?
I would track the following metrics :
- HRV : This is a proxy for your recovery and nervous system state. The FIRST thing to do if you want to perform is not get injured and fatigued. So this is a staple. If your HRV drops, this means you are overdoing intensity or frequency or do not get enough recovery
Sleep : Monitor your sleeping patterns and particularly REM sleep. Same Reasons as above
Motivation to train : Not many people consider that but enjoying your training is important ! If you do not enjoy it ... Chances are you won't grow with it
Track your progress ! Run a 5k, 10k, 15k once in a while in a local competition or something like that and see if you improve. If you improve, chances are you are doing a good training ! And if you reach a plateau, review your training and see what you can change. What got you here won't get you there. If you want to progress again and again you will have to work on things you did not before e.g. strength training, speed training, fix your imbalances, ... Always upping the volume and/or the intensity is not the solution. It will break you at some point.