I believe that 6 minute miles marks the boundary between seroius athletes and mere mortals.
Depends how far you’re running. Doing 6 minute miles for a marathon is way beyond serious athlete, and would get you a guaranteed championship start at London. I had an ambition at one point to do a half in 6 minute miles, and set off at that pace in one race, but it all went wrong after about 5 (realistically I wasn’t in that sort of condition)! Though I have managed sub-6 pace for a 10k a few times (if I’m allowed to willy wave on here, 10k PB of 35:30).
Realistically, if you’re not training seriously for running, sub 8 minute miles over a reasonable distance is pretty decent. Though comparing speeds with others is to some extent a bit pointless – unless you’re racing, just compare your own times and maybe aim to get quicker. Also not every session should be going as fast as you can – that way lies ruin.
When I ran regularly I used to find that I could put in respectable times for short distances (1 mile in 5 minutes, 2 miles in 11 minutes) but anything over that I’d struggle to get much below 7 minute miles.
The guys I could ride and swim with in the tri club would be putting in 33-36 minute 10ks whilst I only once got below 40.
I assume that must be down to genetic factors?
Maybe – though physiologically there’s not a huge difference between 2 miles and 10k, so more likely just that you either weren’t trained to run that fast for that long, or that it was simply a mental thing. A 5 minute mile is pretty serious – that’s as fast as I’ve ever gone for 1 mile, and I’ve given my 10k time above! Though actually even the slow down from 1 mile to 2 seems excessive, given my similar 1 mile time but that I could run 2 in ~10:40.