Wow - a lot of good and useful information from everyone. Thank you!

To answer several questions posed of me, I’m a 67 yo rider who primarily does road riding. I recently bought an Apple Watch and several times I’ve gotten HR readings in the 175 range during a ride. I’m not doing anything heroic, no intervals, no climbing. Worst case is trying to maintain a 15-16 mph pace into a headwind.

Using the 220 - age equation my max HR should be 153. Are those 175 readings real? I just thought I should better understand what my HR is doing, and then look into a systematic training plan that focuses on HR to build stamina.