My own take on anyone who says "Hard work is its own reward."
"If hard work was its own reward, Sisyphus would have job satisfaction."
I certainly get that not all of the rewards of hard work are material or even immediately obvious. But you out to get SOMETHING out of your labor, be it increased strength, skill, experience, knowledge, or even just the satisfaction of a job well done.
If you are getting NOTHING out of it, that isn't being diligent, that's being stupid. Be willing to work hard when necessary, but never work harder than you need to to get the job done properly.
To borrow a quote from Scrooge McDuck "Work smarter, not harder."