Ron Daws, the author of Self-made Olympian - a Lydiard guy and someone else of whom I forget the name of at this moment came to the conclusion that your average male who puts in the training, probably 10 years/10,000-like scenario can run about 2:30.
I tend to agree. 2:30 is fast when you look at the spectrum of times in most marathons sub-2:10 to 8:00 (or 6 or 7 hours).
But the difference between sub-2:10 and 2:30-ish can be a massive chasm, depending. I personally think a healthy young, well-trained male running 2:30 is much closer to the 3:00 mark than the sub-2:10 mark physiologically speaking.
I have seen men and women, that I have observed, coached, or heard of who have run nothing over two hours on the roads for a long run and bang off something respectful like 2:30-ish or for women sub-2:50. And I have seen some jumps in performance for people running new higher volumes and long runs of 30 miles or unique long run days, like cycle 10K to start, run 25 miles, pool run for an hour, cycle back 10K to home and get on with their day mowing the lawn and repairing the fence, go for a good walk after dinner.
I think some bodies are not built for fast marathons. I have the humbling experience of training with people who asked me a lot about Lydiard and happy to impart, I became sort of their coach/advisor. Every single one of them started out as a newb and I was the experienced guy and all of them ended up being faster than me.
I love distance running and did it, but I believe I was built for 400/800m.
Anyway, I always tell people the Lydiard line, "champions are everywhere, they just need to be trained properly." And then go on to suggest that you may have a champion inside you, but because you have never trained, you just don't know it and from your current perspective find that difficult to believe. I can't tell how many smokers, drug-addicted, obese or simply sedentary people get hooked on running and become yet another person who says, "I wish I discovered running when I was young," because they know they would have made a varsity team, or national team or been sub-elite sort of thing.....
Not sure how this fits into the current exchange, but I think 4:30 may only become 4:00 and they may be excited by that, gunning for 3:59 and 4:30 could become 2:30....all 4:30s (sort of speak) should be encouraged to run if they like it.