In my current long commute, I see a number of slower runners/joggers out in the morning. One thing that is apparent to me is that regardless of the fact that they are moving at slower paces, their form looks nothing like that of faster runners. In essence, they are not slower because they have lower aerobic capacity, less training volume, etc., they are slower because their mechanics are poor.
This is where I think interval training can end up being a negative factor in training. If you take an inefficient running style and a person does intervals and speeds it up, are they better off? All they've done is taken an inefficient gait and learned to do it faster. They are still expending greater-than-needed effort to run, and therefore they won't be able to reach their true potential.
Watching the Olympics, it's easy to see that the athletes are not just running much faster than the average person, but they are running much differently in terms of thier form and mechanics.
I think some key factor drops off in adulthood that makes running more difficult than when a person was younger. Maybe it's posture or flexibility, I'm not sure. I read somewhere else that if a person takes up running for the first time after age 30, they are more likely to become injured and have difficulty.
I know when I was younger, running definitely felt easier and more natural. I remember in high school, a friend and I ran a 10k race with no training whatsoever. I saw an article about it in the newspaper, I called him up, and we went down and ran the race. We didn't set any speed records, but we managed through it okay. I can't help but think that a key physical attribute was different back then (alignment, flexibility, something besides the fact that I was younger), that allowed me to run with much greater ease.
