"... average speed to aspire to"
@63 or is it 64 already, typically I ride three times a week hour or hour and a half long rides at fast clip to keep just not out of breath and not sweating way too much, keeping the heart beat up but not too high. Health wise, biking for me is keeping aerobic fitness, beside the pleasure of being on a bike machine playing like a kid.
If younger guys pass me, it doesn't worry me (I can always make up an excuse, like if I had the fancy bike like them, if I was clad in lycra like them...
) but when it looks like someone my age bracket or (say) 40+, I like to be competitive or at least try to be. I still like the speed and even when it is not like any racers speed (it never was in my case at any point in life) it feels like I am going fast. At any rate, it is way faster than any casual bicyclist.
How fast one can go very much depends on everyone's life history. Where I grew up, we have the saying, that what you learned while young, in the old age as if you found it... and it is valid for fitness too. I find that my form doesn't improve much even if I take frequent rides, on the other hand, I don't bike in winter and it takes few rides to get where I left off last year.
When I was younger and pushed it, I got sore muscles next morning but now, I can push it and I don't get sore from it, at most I get cramps in legs at night, like someone said above and suggested solution - more drinking and feeding. I should drink more on my rides, my older bike has only one bottle cage but I got newer one now that comes with two bottles and I plan to fill both of them up, come spring. And with age, keeping hydrated is probably even more important then ever before.