1. There is something called take-off velocity. If you don't reach it, you will never get off the ground. You will keep trundling along that runway forever.
When you're trying to break into publishing, you have to work damned hard, at every stage, from the writing itself to the editing, to the querying, to plain old learning, and hopefully someday to promoting your debut. You must strain your engines to their limit at the start of your career, or you'll have no hope of someday soaring.
The one wonderful thing about that hard-scrabble stage, however, is that the runway is as long as you want to make it. Nothing will ever force you to give up.
2. Once you're in the air, you need to keep climbing. All kinds of things can bring you crashing down if you don't reach a safe cruising altitude.
If you stay on the midlist, your publishing house will eventually decline to work with you again. Hopefully, the industry will someday change such that this isn't so ruthlessly true, but right now, it is. Plan to continue working your butt off even after you get published.
3. Once you reach cruising altitude, you can trust in your momentum to help keep you aloft. However, if you shut off your engines, then you are headed for the ground--no exceptions. It may take a long time to realize it, depending on how high you were, but there is no such thing as coasting--only sinking.
Even famous authors don't get to knock out uninspired books for long. A fan base is only as loyal as the quality of the product deserves. Therefore, for as long as you want writing to be your career, you must strive to go forward in your craft, to keep improving and to keep creating books that are great--not simply good or good enough.
Don't shut off your engines until you're ready to land, i.e. retire.