The book Cycling Over 50 might help for someone of your age. By Friels, I think. Others may know more.

Rest is an underestimated requirement for any sort of bike riding at moderate intensities upwards, especially for older riders. At least, that's in my estimation. I know that as an almost full-time cycling commuter and long-distance rider, I NEED time off the bike at some stages of the year when I will catch buses or walk (hence the "almost" full-time cycling commuter).

You are right about the mind and physiology sometimes not meeting. Getting the body to do more is adaptation.

To be frank, what are your goals. Is this just personal improvement or are you aiming for competition or just a faster commute? I know that over the past seven years, there have been numerous plateaus where my "average" speed hasn't increased, then suddenly there will a spurt (it's a bit like kids with growth spurts) where you move to a higher level.

There are some posts on another list where a bike courier related his levels of fitness improvement after starting the job, and the intesity THAT took.

By the way, have you had a medical check-up lately, and discussed any of this with your doctor? It might be worthwhile if you can't sustain a minute of intense exercise, even at your level of activity.