My answer is simple: Go wild. Don't censor yourself. Vomit on the page. Let it flow. Remind yourself you can always go back and delete. You'll certainly be able to go back and improve.
If your first draft contains paragraphs that don't fit the topic, delete them. However, you may have stumbled on to the idea for a follow-up article.
Write the draft and don't worry about grammar or style. No one has to see the draft but you. You may be one of those logical, analytical writers who thinks sequentially. Let it flow. If you're the other kind and your mind jumps around and you end up with a first draft of 5,000 words and you need only 1/5 of the material, that's all right. Don't censor yourself.
The too-many words writer needs to learn to cut, cut, cut. I call that the fat writer. By contrast, I call myself a skinny writer (I am thin, but that's not the reason).
As a skinny writer, even when I let it flow, my first draft reads more like an outline than a full article, but I've written my concepts and major thoughts. If you're a skinny writer, you'll have to add details or information to bring readers into your way of thinking.
When I write my first draft, I vomit on the page.
I will go back later and clean up the mess.