Back in the late seventies, I rode my Motobecane Grand Record (a real Motobecane) to work, home for lunch, back to work, and home again at the end of the day (2.5 miles each way with mild hills and flats and one moderate hill just before my work place). I did this for 4 years, 5 times a week, along the same road except for the dead of winter. Bicycle computers did not exist and yet I clearly developed an ideal cadence and improved it over this time period. Of course back then, I didn't think of it as cadence; I used the term, "pace" to refer to my peddling. I remember becoming intimately aware of what I had to be doing along the road to be prepared for what was coming next. It was a great feeling.

I used to have this great book called, "Two Wheel Travel". It had a nice discussion of peddling and if I remember right, the ideal RPM back then was one RPM for each heartbeat as a rule of thumb. There is a point where you reach a sort of equilibrium between peddling and cardio. Lots has changed since then with all the gears available now. The book was written with 10 and 12-speed road bikes in mind.