In reply to goldmember:
I was a 4.10 miler when I used to run 8s and 15s on the track. Like most sports, when you start squeezing the margins you really want to have the perfect body shape. And I didn't. I was about 12st 5lbs, which is just too heavy for someone who's 6 foot. My body fat was super low but it didn't matter, I was lining up against guys 2st lighter than me (for the same height), and that just doesn't work.
Someone mentioned running 200 intervals. I used to do stacks of those with a good friend in 28secs with very little recovery between, and I found that pretty manageable. I ran 6 days a week, a mix of speed work, hill sessions, intervals, 10ks etc, as well as half marathons every week, however that extra 10 seconds that I needed was still a world away. In my own opinion, the actual physical work you need to put in is hard but not Olympian hard. But you really need that ideal body shape!
PS. I heard the other day, I think it was in the build-up to Mo's 10k, that he could run a 49sec 400m at the end of a hard training session, and that he's recorded last laps of 51secs and 53secs in major 5,000s and 10,000s respectively. If you think of that turn of speed, running a 4 minute mile doesn't seem so hard
