My sense is he did a lot of work in parallel and let the world find itself among many mediums.
It's a pretty well-known thing that he was a linguist first, wrote the hobbit to read his children as a story that fit in with the languages and mythology he'd already developed, and only ever considered publishing them because his friends (including C.S. Lewis) badgered him about it.
The world of the Lord of the Rings exists solely for the purpose of providing an explanation for the evolution of his made up elvish languages, which was the real interest of this professor of linguistics. That it was published at all was an accident of the fact that people liked the bedtime story he wrote for his son (The Hobbit) which drew inspiration from his linguistically-driven worldbuilding.