From Seth Godin's blog, a post titled "An endless series of difficult but achievable hills
". It's a very short read that contains the following:
"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."
It strikes me that this is a good description for the sweet spot in martial arts practice (or really any practice), where you are getting the "training effect" from what you're doing: enough challenge to make a difference, enough progress to have faith to keep you going. But knowing when you're in that sweet spot (or conversely, knowing when you aren't, and what side of it you're on) is a gift.