What is up with this? What’s going on inside my body?
You’re getting fitter.
Specifically: if you want the whole, detailed explanation look up “physiological adaptations to aerobic exercise” or something to that effect in an ex phys book. Quickly: increased mitochondrial density; increases in the enzymes that catalyze metabolic reactions; increased stroke volume; changes in the nervous system result in the lower HRs; increased ability to utilize fat as a substrate. etc
