Somehow I don't really buy it. Seems to me it is more a management problem. Looking at sports, I suspect simply running 10 hours more every week than the competition won't automatically make you the winner. Rather, there is a system to getting the best effect from training (when to take breaks, what muscles to train when, what to eat, ...).
If you are stressing out with working harder and harder, perhaps you lose the time for having inspirations and seeing the important things to work on.
Working harder does not mean being stupid. I don't understand why people keep pretending working harder is somehow how inversely related to the quality of your decisions. Your IQ stays the same, you just work harder.
Let's take your example. Working harder doesn't mean naively running 10 hours more a week. It means doing research on training systems, consulting with coaches, monitoring your diets, revising workout plans, etc. Having been involved with sports all my life, I can tell you that this stuff matters, you can always do more of it, and it's work.
Sure, I am not against working harder, just against the simplistic "work harder". Also I still think it is a management problem and breaks are important.
If you are stressing out with working harder and harder, perhaps you lose the time for having inspirations and seeing the important things to work on.