[nradov]

That's not how any of this works. Exercise causes some oxidative stress but this isn't a problem in practice. There is only a tiny fraction of committed athletes who over train to the point where it becomes detrimental; most people simply don't have the time or discipline to get anywhere close to that point.

https://doi.org/10.1016/j.jshs.2020.04.001

As for heart rate, going above threshold is never comfortable. But it's a good pain and you can get used to tolerating it for occasional high-intensity interval workouts. Over time this will decrease your resting heart rate and reduce the risk of a major adverse cardiovascular event (MACE).