In the book you finally nail down what is "ok cardio" and what is "chronic" or "bad" cardio - under vs over 75% of Maximum Heart Rate, respectively.
As you've even said, 75% isn't hard and fast for everyone, and that some trained athletes it could be 80%. However, there must be a line that you've identified, and I think I can guess what it is - lactate threshold. Yes?
I'd love to get a more technical fleshing out to this than what is used in the book - admittedly aimed at the layperson, not athletes or people who've done their physiological homework.
Lactate threshold is the point at which you begin to accumulate more junk than your blood can clear out. That's a gross oversimplification, but in a nutshell it means you can exert just above your lactate threshold for 45 minutes to an hour before collapse. For many people using their 10K level of effort is about right. Definitely not mile or 5K speed, and definitely not marathon.
Your thoughts are greatly anticipated. :-)