I read this article by former triathelete Mark Allen a few years ago, and I still find it interesting. Allen found that by training at a slower pace (keeping the heart rate below a certain level), that he slowly got faster and faster. Others, like Phil Maffetone, have written about this phenomenon as well.
I think this type of training actually fits in within a Paleolithic-type exercise program. Mark Sisson has written previously about the dangers of constant high-level aerobic training. However, he does recommend low-level aerobic training mixed in with some anaerobic work.
The way many people normally run/jog, it is not performed at a low heart rate. I feel this may not be because they are running too fast necessarily, but due to poor running form. In other words, running with poor form requires a lot of oxygen, even if the running is done at a slow pace.
In contrast, I think a person who is running in a very efficient manner can run distances at a low heart rate. I will call this "trotting", just to differentiate it from the typical jogging style. Trotting represents efficient, biomechanically correct running, and as such requires low levels of oxygen at slower speeds.
I've written before about how some scientists believe that the aeroic metabolim underlies basic health. While interval training can improve aerobic capacity, it may not be the best method. As Mark Allen wrote about, interval training worked for him only on a short-term basis. For long-term improvement, he engaged in easier, low heart rate running.
