I wonder if there is some underlying lizard-brain thing going on here.
If you "know" you are physically unfit you are quick to anger and aggression because you potentially need to act like that to not need to rely on physical fitness if it came to needing it? I.e. you need to deter others through your aggression rather than relying on fitness if it came to a fight?
Or alternatively the other perspective if you "know" you are fit you can keep the stress hormones low safe in the knowledge that if it comes to it you've got the fitness to handle a fight?
...Or perhaps none of that!
Interesting.
I've had the same idea. Especially regarding anxiety. You start getting anxious and scared of everything, because your brain knows that your body is out of shape and incapable of dealing with stuff if anything happens. If you can't deal with any problems, then you must constantly be on lookout for them so that you can avoid them.
Or even more lizard brain: if you have more cardio capacity/fitness, your body actually FEELS better, so you feel better emotionally and cognitively.
If you have lower capacity, your body feels bad and this is reflected in your emotional and cognitive state.