[layman51]

Overdoing it is a real risk, I learned recently. If you work out too hard, you actually produce free radicals. The tricky thing is that some people can get used to higher heart rate so it doesn’t matter if they are working out hard consistently because they are used to it.

[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).

[switchbak]

You need to ramp up, of course. Given you're training at an appropriate rate, and you're doing sensible things - you should be fine. By sensible things I mean: short and intense weight training, intense cardio sessions, and not-too-long endurance sessions (eg: < 5h). More endurance is fine, but it can be very fatiguing, especially for the older athlete. If you ramp up to those over time, you needn't worry about "free radicals" (that's an oversimplification).

Recent research highlights a number of things, but what sticks out is that it's important to maintaining muscle mass because we lose a bunch as we get older, and loss of muscle is a leading morbidity factor.