Every long piece of writing has a false start buried under it. Usually several. The habit worth building is not avoiding them — it is recognising one early and putting it aside without ceremony.
A false start feels like writing. It has sentences in it. What it does not have is a reason to continue, and no amount of revision installs one after the fact.
The tell is simple: read the last paragraph you wrote and ask what has to come next. If the answer is "something", you have a false start. If the answer is a specific thing you are slightly reluctant to write, keep going. That reluctance is the piece.