Despite your concluding statement, there is something called "bonk training". Basically, you exhaust your glycogen stores as much as possible before beginning

Despite your concluding statement, there is something called "bonk training". Basically, you exhaust your glycogen stores as much as possible before beginning your workout. So when you start running/biking, your body is in a similar state to just after hitting the wall in a marathon or several hours into a long bike race or triathlon without eating. Since you have insufficient glycogen, you just can't do more than 70% your max heart rate. It is reputed to produce advantageous training adaptations when done in addition to normal training. But if you aren't on a keto diet already, that sounds like a great way to simulate the worst tortures of Hell. You probably won't even be able to push yourself hard enough to faint, but you will wish you had. There's a good reason racers down that glycogen gel stuff during their endurance races now.