I've seen the reports that extreme cardio, especially performed over long periods of time, can be bad for your heart and cortisol levels but I'd like to know where I should draw the line?
For example, if I work out for an hour on my rowing machine (yes, not very primal, I know but I really love the motion of rowing) and I keep my heart rate to 70% of my max does that keep me out of the cortisol danger zone? If I breathe only through my nose (keeping my mouth closed) does that keep me out of cortisol hell? If I do interval training for 10-15 minutes three times a week, does my body have sufficient time to recover?
The only times I've ever been as lean as I'd like to be have been when I've added cardio to a relatively low calorie lifestyle. Yes, I know everyone says that leanness is mostly about diet but, for me at least, exercise has a sufficiently noticeable effect.
I weight train three times a week. I eat mostly paleo. I don't pig out. I'm wanting to squeeze out that last bit of fat without hurting my heart.
According to Charles Poliquin, a world renowned strength coach, cortisol starts to ge seriously up after 20 min of cardio, or one hour of strength training. He never prescribes cardio to any of his athletes, since cortisol is essentially catabolic, and according to studies, makes you lose strength.
For fat loss purposes, he advocates strength training to build muscle mass to increase the body's calorie consumption. But if you really like cardio, interval session under 20 minutes should do the trick without compromising adrenal health.
From a chronic perspective, I look at other symptoms.
If I'm in weight loss mode, and I gain weight after a week of strength training and exercise, I cut back on the exercise until my weight drops or stabilizes again.
If my sleep patterns start to get disrupted under the same conditions, I also cut back on the exercise until sleep patterns get a little more normal.
And so on, with sex drive, appetite, core temperature, etc...
There's no hard and fast answers. generally, keeping cardio exertions below 30 minutes, or an hour at most, is recommended. But for some people just trying to walk that much may be too intense. I can hike all day, and I'm not going to let fear of a little cortisol restrict me to only ever walking one hour. On the other hand I wouldn't want to run even that long regularly. But then I play sports often for a couple of hours at a time, which is cardio and interval type work. But I don't exercise primarily as a over-complicated system of cortisol control. Maybe the enjoyment counteracts the increased stress. Maybe my gut feeling to take a break every few weeks helps keep things under control. Maybe it really doesn't matter all that much.
I would say that 1 hour at 70% is definitely in that bad range. Basically, to prevent the cortosol, you need to do something quick and hard that stimulates the growth hormones and such, and then you need to give yourself sufficient time to recover so that you're not stressing your body.
All those HIIT/Crossfit proponents (me included!) who do HIIT 4, 5, 6, times a week are not doing their body any good. HIIT 1 or 2 times a week is what you want, that way you have time to recover.
The long-slow 70% efforts you're doing on the rowing are just like slow running, or cycling. You're just doing this constant low-level damage to your body for a whole hour. That's definitely going to cause some bad stuff
That's not to say you shouldn't do it if you enjoy it. But it's not "good for you". If you love to row, do a 2K once a week (that's only like 7 minutes) and then do a 500 once a week (1:30) and you're done with exercise in less than 10 minutes a week, and I bet you'll see great adaptation to those stressors.