Are you familiar with FTP? Functional Threshold power? It’s the amount of power you can put out such that the amount of lactic acid (the “burn”) your produce in your legs is less than or equal to the amount of lactic acid you can clear. (I think, could be wrong)
FTPs indicate your “blowup” point. Most people can hold their FTP for about an hour. If they go over their FTP, they usually burn up pretty quickly.
But I think you’re more interested in your aerobic threshold. That’s the amount of power you can put down using just fat and air (I think). Any amount of power under this threshold is “easy” and doesn’t really fatigue you at all.
TrainerRoad should have a lot of stuff about this. But my uneducated advice is to try some polarized training. 80% really easy and 20% really hard. Don’t do a bunch of medium pace riding. That fatigues you but doesn’t force much adaptation.
If you don’t have time to ride 150-200 miles a week, you can try riding your 80% fasted. This will force you to burn fat and air due to a lack of glycogen in the muscles. Obviously, you should not do hard riding in this state.
Hopefully some other people can chip in.