Though I am not an expert in physiology or in training, I have studied (for fun) elite runners training as well as much of it from over the past century. One of the common themes is moderate to high mileage. There are some but very very few runners especially for mile up that experienced a lot of success without hitting around 70 mpw for at least some of the year. Depending on who you are you may consider this low, moderate or high mileage....Even today most (not all) highly successful 800m runners spend at least some portion of the year running 50mpw + when considering the distance of the event is relatively high in comparison.
Now saying that just about everyone that I have studied that has been highly successful in our sport also does faster to moderate paced runs during train(duh). I do not know of any runners off the top of my head that have only did slow mileage without ever hitting at least marathon pace or faster during training (I’m considering anything that touches marathon pace or faster to fall within moderate or faster spectrum). Some obviously hit moderate to fast paces more than others. Some have found high amounts of success doing as little as 1 moderate to fast paced efforts (tempos,fartleks,intervals) every week. Some do some sort of moderate to fast paced efforts almost daily...
Saying all that we understand that even slower paced running promotes physiological benefits that are beneficial to distance running events without a high amount of stress.
Again by no means am I an expert but the general idea is to create a sustainable systematic approach that will make a runner faster without a high risk of injury or burnout.
From a physiological perspective and practically speaking moderate to high intensity training though very beneficial also promotes the greatest risk for significant injury and in general requires more recovery before the body is ready. LSD is a more sustainable approach that you can do day in and day out without a high risk of injury while still applying stress to the body that will result in running faster.
Sure you can get injured but it typically happens over a longer period of time and typically there are warning signs prior and often the injuries are less significant vs HIT where it often happens instantaneously with little to no warning signs.
In my experience I was more high intensity low volume in high school and ran ok (16 min). My first time doing higher mileage I doubled what I did in high school from 25-35 to 50-70 mpw consistently for 1.5 to 2 months. During that span all my runs where at 630 to 830 pace and I only did 2 workouts where I hit paces faster than my everyday pace. As a result I dropped my pr 30 secs 6 weeks in while going out like madman and only 18 secs slower than my mile pr during that 5k.
Obviously there are huge variants but generally speaking there are very very few exceptions of individuals that have run what we consider elite without relatively moderate to high volume.
My guess is that if you where to put together a study that included every runner that has ever run relatively fast let’s say sub 4, sub 14, sub 29:30 my guess is less than 10% achieved this without hitting 70mpw + for atleast a portion of the season.
To conclude, its hard to say there are absolutes and there are always exceptions but to give your self the best odds at being successful in our sport from a training perspective (I’m assuming you possess the talent). You need both relatively higher mileage/volume along with occasionally running paces that would touch the moderate to faster section of the spectrum. Those that choose to ignore history and do only high volume or only high intensity give themselves the lowest chance of being successful assuming they possess the genetic talent to make it far in our sport.
Sorry for long winded, poorly structured and ungrammatical response. It’s like walking on eggs or land mines around here.