This sport is my life. I am certainly not the average guy and I was thinking that being able
to run a sub-19 5K in my 50s was pretty good. I ran nearly every day at that time and was a long-time coach.
I would have had a very hard time doing 60 seconds for the 4 0 0, unless I trained exclusively for that.
The average 40 year old could not break 20 for the 5K. There are a few naturals who could do it.
When I was part of a running club that was full of average, middle-aged people I took the opinion that with the right training all men would be able to hit 20-mins, and all women about 23-mins. The middle-aged people I saw doing these times really weren't that physically spectacular - they'd just been committed to running at their club for some years.
Since then I've begun to conclude that most people are not willing to do the training on a consistent basis that would allow them to do this. Most people always want to just run fast OR slow (depending on their preference) on 2-3 days each week when it suits them. The lack of an organised training programme to develop the full range of aerobic-anaerobic holds them back.