On metabolic vs nueromuscular adaptations...
let me synthesize what I think you both are saying with what my personal experience was...that said it shouldnt be taken as a general statement since every runner has their own set of talents/genetics/strengths/weaknesses...
I had an excellent VO2 max (tested at 80) but very less than average mechanics (6'3" with 31-32 inch leg inseem which is short). I had very good explosive power (37 inch verticle) and a light "frame" --ie I could keep power and weigh only 157-160lbs.
I believe that there is a general level of distance training that allows you to perform more consistantly at and recover quicker at your VO2max...you are "training to train" so that you can handle 4-5 hard sessions per week and doing all the mechanical work at faster paces to teach your nervous and muscular system how to hold top speeds longer. One enables the other. If you dont have a general base of 70-80 miles per week at the elite level is it difficult to recover enough between intervals and between workouts to run the intensity fast enough.
That said, I would say that most of the improvement from training at the elite level is nuero-muscular. I would make this more approachable though. When you are normally growing up, you have a concentration of your fast twitch muscles in your power centers--ie quads and gluts, and a perponderance of slow switch in your stabilizing muscles of hamstrings and soleus. When you see little kids running distance, they do to what feels comfortable--using the more conditioned and least fatiguing hamstrings and calves to pull you through, which is why they all appear to overstride and shuffle. I did this all through HS and was a terrible distance runner -- you cant generate any power or stride length that way...
over time, you learn to train your core muscles to stabilize you, your glut and quads to power you, and your calves are just quick energy return rubber bands to focus the power being generating in your upper legs into forward motion. I remember seeing runners who had broken into the next level actually looking different -- prancing with hips forward and big back kicks...they had trained their power muscles to last. Haile G form...Deena Kastor form..Seb Coe, Hicham...watch the form on Youtube..
This process takes years, drills, stretching, plyos, hills, and many many miles where you practice good form. But it is how you can drop another 6-8 seconds after High School in the 800m for the elites. Mileage definately helps this as well...you should be practicing your form every single step you run. Injuries all happen from exhaustion (either muscle or mental) where you slip back and hurt hamstrings, IT bands, achilles all attached to the adolescent slow twitch muscles...
My personal suspicion...havent seen this address in research but I am certain it could be..