Originally Posted by
ericm979
You should read a book on training and physiology rather than asking a forum, where the answers will vary widely in quality and even the best will be too brief.
I suggest Thomas Chapple's Base Building book as it has the best explanation of the physiology.
More brutally: a lot of people here are idiots. If the OP doesn't want to buy a book (and not all books are good) then he'd be better off googling than asking a forum question. Eg
http://ultracycling.com/sections/art...y_training.php
Interval training at high intensity has been an accepted part of endurance training at elite levels for years now, because it works - as for why it works, there you get into some pretty complicated physiology.