Tolkien made an interesting comment when he tried, early on, to try and explain his new mythology.
He soon came to feel that the composition of occasional poems without a connecting theme was not what he wanted. Early in 1915 he turned back to his original Earendel verses ad began to work their theme into a larger story. He had shown the original Earendel lines to G.B. Smith, who had said that he liked them but asked what they were really about.
Tolkien had replied: ‘I don’t know. I’ll try to find out.’
Not try to invent: try to find out. He did not see himself as an inventor of story buy as a discoverer of legend. And this was really due to his private languages.
-Humphrey Carpenter, J.R.R. Tolkien: A Biography, p.83
The language was what was invented, though still based on a deep knowledge of linguistics. Then the mythology came later as a way of explaining how the words developed (and they were already developed). The language poured a huge slab of concrete for him to build his cathederal upon.