That is an interesting philosophy on setting a goal you’ll never reach. I’ve been setting goals following a 10X method which is basically making them ten times as big. The idea is much like an unachievable goal, it’s not meant to be achieved, but it gets you to be thinking about what you can do to get closer to it as well.
Either way, what you presented is a good way to be getting closer to any sort of goals that you set. That being said, one thing I think is important as well that you didn’t mention is why you are doing it in the first place. Going back to your writing on Fanfiction, perhaps the reason you could write so much back then was not only the habit of writing, but you also had a good reason to write as well. Whether it was your passion for particular characters in a story, or writing fanfiction was something that provide emotional support for you. Regardless, the reason must be something that moves you and is motivating enough for you to pursue and continue to develop, even in times where you stop and take a hiatus.