I love this analogy.
From: Seth Godin
Repeating easy tasks again and again gets you not very far. Attacking only steep cliffs where no progress is made isn’t particularly effective either. No, the best path is an endless series of difficult (but achievable) hills.
The craft of your career comes in picking the right hills. Hills just challenging enough that you can barely make it over. A series of hills becomes a mountain, and a series of mountains is a career.
Sometime our work assignments gives us the hills and sometimes our personal projects provide us those hills.