I'm new to using HR. I'm 53. My current RHR is 45 (partly genetic & medically cleared). I've been sub-40 when well-fit. My calculated max HR is, obviously, 167; yet I can spend ~30 min. in a 160-175 bpm range w/ no signs of overexertion. My HR zone for vigorous training calculated based on heart rate reserve is 130-149. Even for a 40 yo, the top end is only ~160. Also online: "Athletes can reach max HRs of 180-200 bpm." I want to push my cardio fitness. Can I ignore the preceding data and calculations and just probe gently until I start to experience overexertion or overtraining?