No offense, and I hate to be the grammar police, but two common grammar mistakes two words in a row is textbook.

But to be serious... People measure what they can measure and then use rules of thumb to close the gap between numbers they can measure and those that they can't. You likely have your usual routes you ride and know your usual average speed and some perceived effort/observation of conditions to make the correlation between the number on your cyclocomputer and your form.

A hill climb, timed between signposts or mailboxes, is another example of how to track your fitness.

Rules of thumbs about number of miles to ride in a week, or number of hours to spend on your bike, or cranking out tempo in the 53/18... all these are imperfect gauges to measure form, and they all work provided the rider has a lot of experience on the bike and with their body. Every new measurement instrument just makes everything more and more exact and flexible. With a heartrate monitor, you don't have to guess at the wind. With a powermeter, you don't have to climb the same hill every week.

Anyway, musings, I guess. But overall I agree, the old ways still work, but they are not as precise and are harder to implement. And the OP's desire to shortcut experience by asking other's about their average speed is a bit asinine. If all you have is average speed, pick a route, ride it a lot, and track your average speed and your observations about conditions and go nuts. But you won't learn anything by comparing your average speed with someone else's average speed.