As I was running the other day in the gym, I felt I wanted to quit before reaching the goal I had set for the day. My legs were tired, I was breathing hard, and I was hot and sweaty. But as I evaluated each of the these three factors in sequence I realized: my legs were not that tired; I was not exerting myself unduly; I was not all that hot. So the weak link in the chain was my mind. So of course I continued to run, ignoring some rather minor discomfort. Because if I had a weak mind, then that in and of itself was not going to stop me! I thought, because the mind itself isn't hot and sweaty and tired.
So what I thought of as the need to have an iron will is just the opposite. A strong mind is fully aware of its own weakness but doesn't let that weakness get in the way.