Yes, it's true that long runs and high mileage encourage the production of mitochondria and promote the growth of capillaries. The question is WHEN WILL THIS TRAINING WORK. If you want to build a house of 10 floors, when you arrive at the top you cannot continue to build. If you want to be more comfortable, YOU CAN ARRANGE SOMETHING INSIDE THE HOUSE, but you cannot touch the structure, that at the moment is finished.
In the history of mythologies, there were many Coaches and Scientists that used very long runs for the CAPILLARIZATION. The first was Van Aaken, great coaches as Frassinelli from France used to start every season with 3 hours of long run at 5.0 per km (Jazy, WR holder of 2000m). But look at the value of the performances.
Already in 1964, somebody (Snell) was able running 800 in 1.44. In 1979 Seb Coe ran 1.42.33. Now, nobody is able to run so fast. The Kenyan record of 800 is 1.42.24 from Sammy Koskei in 1984.
In 1500m, 30/25 years ago we had a lot of European (not TOP champions) able running under 3.32 (Wessinghage, Hudak, Deleze, Abascal, Gonzalez) and somebody under 3.30 (Auita, Cram, Coe himself, Maree). Athletes from GBR able running 3.33 (Robson, Moorcroft, Rowland) had to find fortune in other events.
In 2008 for the first time in 30 years NOBODY RAN UNDER 3.31.
Instead, in 5000&10000m we continue to improve the best average, and in HM and Marathon everything changed.
Why this, if not connected with the use of LONG FAST RUN, able to give stimula to the body.
Dont forget that TRAINING IS THE ANSWER TO THE PROPOSAL, NOT THE PROPOSAL.
Without stimula in some direction, there is no answer, and the performances go down.
The repetition, year after year, of the same training (running 2hr30 instead 2hr15 at the same slow pace is not a different stimula)brings the athlete to a situation of non progress at the beginning, and of decrease of his qualities in the second time.