Let every man be occupied, and occupied in the highest employment of which his nature is capable, and die with the consciousness that he has done his best.

Working at the top of what you are capable of is a different thing from working hard. Plenty of effort goes into tasks that anyone could do, and a person can be thoroughly busy for a decade without ever being stretched by anything. The satisfaction being described here comes from having spent yourself on the hardest thing you were actually fit for.

Look at where your best hours currently go. If the sharp part of the day is being handed to admin and the difficult work gets whatever is left at five, swap them for a fortnight. Most people find they were not short of capacity, only spending it in the wrong order.