I bike mostly for pleasure, But even so I have a system to keep a reasonable pace, Whether I'm fighting wind @ 10 kph or regular cruising. I know a comfortable cadence for me with nominal effort is 68 rpm, Ranging from 60 at a relaxed pace to 80 with higher effort.
I made a chart with excel showing speeds in gears at 68 rpm. I have memorized the 6 of them or so that I use mostly and when I find my speed, As indicated by my cheap bike computer is much lower then my 68 rpm calculated for a few minutes I switch gears. M7 for example is about 23 kph with a moderate effort. If while maintaining the same effort it drops to 20 kph I'll drop back to M6 which is centered at 20 kph.
This way my legs are more likely to be fresh for hills or wind. And my breathing recovers much more quickly then my legs. Its better to err on the side of spinning slightly to fast then to burn out our leg muscles.