In reply to Paul Atkinson:
> The question is at what stage, if any, it is worth someone like me doing specific "quality" sessions like hill reps?
My reply would be
"at the stage when you feel like doing it".
Will more structured training get you fitter than "just doing for a ride"? Almost certainly.
But the key question is: will you enjoy/get more satisfaction out of doing structured training than "just going for a ride"?
On my bike I almost never do structured training, but I ride over 100km regularly, over some big Spanish hills, and usually at an average of over 25kmh. I'm quite happy with that, but to a more serious cyclist such numbers are peanuts.
I am also inspired by personal challenges, but the only time I ever 'structured' my cycling was when I was training for a triathlon, and frankly I bored myself to tears, and afterwards reckoned that I probably needn't have bothered. Right now I'm training for a couple of sportives this summer, and that training involves riding as much as I possibly can, over the biggest hills I can find, and as fast as I possibly can. And I'm really enjoying myself!
It sounds a bit twee, but motivation is the most important factor. So by all means start doing some 'sessions', but if you don't find yourself enjoying them, then stop!