This is the huge Occam's razor in the room.


Word to the wise for you hard-core endurance athletes: As you get into your 40's start respecting your textbook max heart-rate.

- Keep your cardio mostly in the green/yellow zone.
- Limit red/max HR to intervals of a minute or two tops.
- Don't push intervals much beyond your max HR.

If you really want to compete seriously, go get a treadmill test to baseline your *actual* max HR (which does vary by individual) and then respect that. It will probably be higher than the theoretical age-derived max, if you were a decent athlete in your youth.