jwadleig:
It's time to start pushing yourself harder. You will never raise your anaerobic threshold or develop aerobic power you need by riding at lower intensities.
At a minimum, ride 3 times a week at 80-85% of MHR for at least 30 minutes, preferrably an hour. That's a bare-bones minimum for any competitive rider for maintainence of exisitng fitness, but not hard enough for complete development of racing fitness.
80% of MHR is really nothing. Well-trained riders can hold around 92% of MHR for an hour. Time trials are usually done at around 86-92% of MHR. If you want to TT at 40kmh/25 mph, you will have to do a lot of work at these intensities.
Build up your aerobic capacity from the bottom up rather than the top down, meaning don't kill yourself with super-hard efforts.
Start doing several 10-20 minute efforts at around 85%+ of MHR 2-3 times a week. Start with 2-3 x 10 minutes. In time your threshold will rise, and you will be able to maintain higher pr's, pr's you will need for proper fitness development.
Equip your bike with a cadence meter and get familiar with what gears you normally use. Aerobic conditioning is stimulated to the maximum at cadences of around 100 rpm, even though you will probably TT at a lower cadence.
For example, if you hit 85% of MHR in a 39x15 now, in a few weeks or a month or so, you will need a 39x14 to hit the same heart rate. Keep doing this until you run out of gears.
I like to "work" a given gear usually between 90-105 rpm. When I get to the point where I can hold 100-105 rpm consistently, I move to the next harder cog and start all over.
It's easy to stagnate (as you have done) at a certain speed. It takes determination, courage, and commitment to start pushing bigger gears/higher cadence. Bigger gears/higher cadences will force you to work at the higher intensities you need for TT/racing fitness.
At your age you can easily hold 25 mph for an hour with proper training. There's a guy in the UK that was 57 years old and regularly TT'ing over 30 mph/49kmh!!!
Good luck!!!
