What’s the big deal about training at a lower heart rate? Doesn’t training at a higher heart rate just mean that I’m training more?

No. That would be true if the body only burned fuel in one way (or only one fuel). In reality, the body is more like a hybrid vehicle in the sense that it burns two different fuels: fat, thanks to the aerobic system, and sugar, thanks to the anaerobic system. Sugar is very powerful but it runs out very quickly, while fats are less powerful but last a lot longer (since we have far, far more fat calories in our bodies).

The issue is that aerobic fat-burning tapers off at relatively higher intensities—at a higher heart rate—while anaerobic sugar-burning increases. This means that if we train exclusively at a high intensity, we’re only developing the sugar-burning machinery but not the fat-burning machinery.