I guess you can probably call this cardio experiment Part II.
Fred and I have been talking back and forth or at least probing between the two of us on intensity of exercise, duration, etc. Don't let anyone tell you it's an exact science or that they know exactly the formula. The fact is, no one does. Which is why there continues to be studies coming out. I can tell you that you can wear your body out. Why wouldn't you be able to? Think about it. What I can tell you is you need to recover. And, I also believe there are different recoveries associated with the body. The nervous system is a whole animal that gets neglected because you cannot see it. But, it's the electrical system that drives the body including the muscles.
Overtrain or tax that nervous system and no matter if your muscles feel fine, you won't be firing on all cylinders. I did an experiment yesterday and until I follow through with it I won't be able to draw any sort of conclusion. I have not pulled my sled all winter. I jogged instead. I could easily jog a mile even at once a week without feeling too taxed at the end. Yesterday, I did two up and backs in my normal sled run and was sucking major gas. I was just walking fast. What gives?! The intensity of effort was much higher muscularly. No doubt. What I am going to do is drop the jogging and go to sled pulling only again. Then, go back and try and jog/run a mile. I'll give it a month. Now, if anyone cares to remember, Arthur Jones decades ago said that proper weight training done with minimal rest is the ticket.