I'm 54, ridden for 22 months, and was previously a couch potato.
I rode 4200 miles my first year (mostly on a Trek comfort bike) and will probably have 6500 or more the second year (on a Bianchi Volpe touring bike) mostly because I can ride year-round.
I went through what I guess are normal progressions - started really slow (first ride averaged just over 8 mph!), get faster fairly quickly at first, hit a plateau, finally improve some more, etc. My normal ride is 11.2 miles and my best average speed was 18.05 mph - but that was way back last March! Further, I have had trouble even getting close - 2nd best is 3/10 slower and 3rd best is a couple 10ths slower than that.
During this long time where I was stuck, I did make good progress on my rides where I wasn't pushing super hard so I was satisfied. But then that seemed to suffer during September.
After a 3 week vacation, I was really slow (only 13 mph on the first 4 mile section!). What was bad was that it didn't seem to improve much over the next couple weeks.
Talking to someone, they said it might be my diet (true, I don't eat like I should since I hate veggies, but that didn't explain why I was doing worse). When he found out I had been riding about 20 months, he said I had probably depleted my stores of trace minerals and now it was catching up to me because of my diet.
So, I went to the local GNC store and got a mineral/vitamin supplement ("MegaMen").
It seemed to help right away, significantly, but after that progress has been more gradual.
But now I seem to be stuck again. I have reached 17.2 average once and 17.0 a couple times, but there are times, like yesterday, where I'm well rested (2 days off) without a headwind, pushed really hard and only averaged 15.7 mph!!!
Sometimes I don't push quite as hard and actually do better, but it still seems that I'm not doing what I should.
Some things I've noted:
Coming home, I often take my time - yet I'll get just as tired as when I push a little. Also, when taking it easy, I find it hard to pick it up part way back - its as if my muscles have adapted to a certain pace and won't cooperate if I want to change. Sometimes it feels like something is dragging the bike back (but I check and everything is fine).
While usually my total ride is 26 miles (half before breakfast and half after), I was, until recently, doing 36-40 regularly (still split though) and I have done 100 once, a couple of 60+, etc, but I haven't done those for some time and never did them regularly.
I don't have a heartrate monitor, but my resting rate is usually 56 bpm. I've checked a few times when stopped at lights and found that when pushing hard, the rate is about 144. On less aggressive rides, it will be more like 108-124.
Other than knowing the calculation of 220-age (166) for maximum heart rate, I don't know what mine really is.
Oh, one other thing that seems strange to me, especially since I'm not getting the speed I think I should, I used to get a burn when pushing really hard very quickly and I couldn't put up with it for more than a few seconds. Now I don't seem to get the same feeling, though whatever it is I get is uncomfortable and I have to back off fairly quickly too. Seems to me that should mean I could go faster, but something isn't right.