Okay, I've notice some things on my rides now that I have my heart monitor working.
When I measure how fast I can ride, I mostly base it on the first 11.1 miles of the trip - 3.5 miles up and down hills and the rest pretty flat.
I started riding 4 years and one month ago (at the age of almost 53).
On my first bike, the best time I ever did was 41:02 on a trek comfort bike. Also, the route I took back then was a bit longer and probably less efficient too (I think 11.9 miles) after 8.5 months and around 3000 miles of work.
My second bike was a Bianchi Volpe touring bike. This almost immediately reduced my best time by a hair over 2 minutes. Eventually I got the best time down to 36:50 (18.04 mph) after 4 more months. But after that, in spite of accumulating over 11,000 miles on these first two bikes in 2 years 1 month, I never broke the record again. In fact, I went through a period where I seemed to be getting slower! I started taking a vitamin/mineral supplement which seemed to help, but I didn't set any new records.
Then two years ago, I got a Felt F-35 racing bike. I immediately set new records and every time I really pushed hard, I would be a little over 18 mph. After 2.5 months, I set my last record (34:41 / 19.19 mph).
I have since ridden for another year and 9 months without coming close to that time (and remember that while that was a faster speed, part of the time improvement was because of a couple route changes which eventually changed the distance from 11.9 miles to 11.1).
The only big change in equipment I've made has been with tires. It came with Hutchinson Carbon Comps - pretty light, but after several sets, I gave up on them because they got too many flats. I've since used Conti 4-seasons, Conti 3000, Conti ultra-gatorskin, and presently the new, relatively light weight Armadillo Elites (255 grams).
I've put in a LOT of miles - over 15,500 just on the newest bike in 25 months and over 26,000 since I started 4+ years ago.
I've changed from riding about 12.5 miles before stopping for breakfast, resting 2-3 hours (mostly before eating), and then riding 13.5 miles home (taking it easy going back usually) to riding 36 to 55 miles before breakfast, resting, then the 13.5 miles home.
So, after that long intro, I don't understand why I can't get faster in all this time! My max HR is 183. On the past couple rides, it seems to mostly center around 145, getting higher on those hills if I push it as I did. But after about 9 miles, I'll start seeing the HR dropping below 140 most of the time and after another mile or so, I'll see more and more in the 120's.
I'm not winded, but my muscles just don't seem to want to be pushed harder. Occasionally I try to kick it up, but I run out of strength pretty quick. Generally the rest of my ride (these two had about 37 before breakfast) tends to have the HR running between 110-125 with occasional fluctuations higher.
I have found, on the rare occasions where I have another cyclist pass me that if I try to catch up to them, I can usually at least keep up and maintain that for a couple miles - I assume because of an adrenalin kick, but even after that, I tend to be able to cruise at a faster pace than before the informal "competition".
So, why can't I get faster after all that work?
I've considered that doing so many long rides usually every other day leads me to develop a pattern of being slower so I don't run out of energy. Many of my rides were over 70 miles and over an 11 week period, almost all of them were 63 miles or more.
In other words, have I trained my muscles to be "slow"?
A friend I ride with often who has ridden less than two years is darn fast. Granted, he is just 23, but can push it at 35 mph on flats almost any time he wants to for a least a short distance. He recommends I climb lots of tough hills (which he does a LOT of). I am trying to do one extra/steeper hill each ride. I've also just started to try to take every hill more aggressively (usually on a steep hill, I just take my time, but I thought that might be part of my problem).
Anyway, he thinks I need more strength so the hills would help. The strength thing makes some sense to me because while my HR isn't all that high after 9 miles and I'm not having any trouble breathing, I just don't seem to have the strength to kick up the speed more.
All this is making me frustrated. Plateaus are one thing, but this is ridiculous.