This is the point : how we can define SPECIFIC endurance ?
For a marathon runner able to run at 3'per km (2:06:36), every long run inside 3' and 3'20" has a SPECIFIC impact. The long run inside 3'20" and 3'40" has a SPECIAL impact (it means is the support for the SPECIFIC). Every other speed is in the field of REGENERATION : under the organic point of view, it's useless, also if we continue to use for other reasons (adaptation of the body structure to the distance, adaptation of the mind).
SPECIFIC is everything inside 90 and 100% of the race pace.
SPECIAL is everything inside 75 and 90% of the race pace.
RIGENERATION every speed slower than 75%.
But this, that is true for a marathon runner, is not true for a middle distance running.
This type of athlete has little lactic power (may be not able to reach 8 mml after 600m at his fastest speed), and a "ratio" between his An Threshold (we can see his best time in HM) and his marathon pace of about 5% (example : 95% of 60'in HM is 63' x 2 = 2:06').
If we speak of a runner for 3'30" in 1500m, he needs to have completely different characteristics. He needs to have a lactic power of 16/18 mml if type fast, of 14/16 if type resistant. His VO2 max is higher than the VO2 max of the marathon runner. We are not interested in a very high An Threshold, but in a very high MAX LASS (maximum lactate steady state). HIS SPECIFIC THRESHOLD IS TO BE ABLE TO STAY FOR 15' BETWEEN 10 and 12 mml, for example, without peaking to early.
The question is : which connection can we have between slow basic run at level of 2-3 mml, and this type of SPECIFIC THRESHOLD ?
You speak about Steve Scott, for example. I don't know how long was the run of Steve, but I know that you can't compare his activity with the activity of Peter Snell. How many competitions Steve Scott as able to do in his best seasons ? If I well remember, he was one of the athletes producing more quantity of results during his seasons, and at the beginning of the seasons never was able running too fast. So, his shape came using frequent competitions (quality) and reducing mileage, step by step, during the season.
Don't think O don't believe in mileage. I have athletes running 140 miles per week, and are specialists of 5000m. But this only for a short part of their career. When they already built their "aerobic house", I use a higher percentage of intensity (never going under 80% of their specific speed) and, of course, I reduce their mileage.
I repeat : the secret is to increase ALWAYS the volume of SPECIFIC TRAINING. So, during the development of a full career, if we start with 85% of general mileage, good for building up capillarization and different kinds of adaptation, and only 15% of specific mileage, after 5-6 years we have no more than 50% of general and 50% of specific, and at the end of a long career (something that for example Peter Snell or Herb Elliot didn't have, but John Walker had) we reduce the general to 30%, enhancing the specific to 70%.
This was, for example, what I did with Shaheen. He started with me with very little mileage (less than 60 miles per week) in 2001, we went till average of 120 in 2004 (in winter, of course), and again we reduce to 100 in 2005 and 90 in 2006. However, we continued to increase the SPECIFIC LONG RUN in some single session, till 37 km in 2:01 in 2005 (not in winter, but in July, before winning WCh). So, what we cut was the long slow run, because didn't have any organical function, but on the other side was a way for stressing the body for nothing (tendon and joints).
We have to distinguish between HIGH AEROBIC RUNNERS and HIGH ANAEROBIC RUNNERS. It's clear than we can have advantages in enhancing the volume of training in the first case, not always in the second case.