Part of it is just continued effort, anon. 60 bpm is on the low end of average adult. =< 45 bpm is in the range of professional athletes.
At least in running, time is more important than intensity for heart rate and breathing. A mildly elevated pulse for long periods allows the body to gently adjust. But if you’re maxing out your pulse during exercise, that’s a significant strain that your cardiovascular system can’t easily equilibrate to