Charlie - you taught me a new word, which I should have know being somewhat involved in the genetics of thoroughbred horse racing. I did know 'Polymorphic' as in Single Nucleotide Polymorphism, but did not know Pleiotropic.
I think we do tend to over think of training in terms of heart, lungs and muscular-skeletal systems, and not enough in terms of neurological recruitment, and the impact a single session can have in turning on genes.
5k is about as far as I'd go in terms of being serious - it's actually probably at the outer edge of my best distance, but if I'm fit I can usually run a time in the top 6-8 US track times in the age group (I don't show up on the rankings as I'm English). I did get third in US Masters at 5k a few years back before the 'injury cascade.'
I'd always tended to be a lower mileage/higher intensity runner and even now a steady run in the 7:30 - 7:40 range seems slow, unless I'm dead tired.
Apart from training for a couple marathons in my late 30s (at which I sucked, about 20 minutes slower than my 3k/5k would indicate although I ran even pace) I tended not to do 'long' runs of much over hour. I did get persuaded to work up to 3 or 4 runs of 1 hour 45 min per cycle in my mid-50s, and did get noticeable improvement.
At the moment, I'm doing a 'long' run of about 8 miles; one slow 4 miles the day after the long run; two harder sessions; and three steady 5 miles runs. If I do go longer I might go to trails and run for time - I find on the road I just go way too quick!
Must have just missed you at the track today if you were in Florence. That's a dreadful track. Was there about 1:00pm and did 5x1000 with 1:45 jog in average of 3:56.5.