Back when i wrote a lot, i often did something like this. I used a program called Acta [1], which was described as an outliner. Basically a text editor where paragraphs form a tree, and nodes can be collapsed. I'd start with the high-level structure and recursively flesh it out, and just keep doing that until i had nodes at the level of individual sentences. Then i'd just write a sentence for each node, and concatenate them. Often i'd take a shortcut and just write a whole paragraph. But a lot of the time, i wouldn't - a difficult passage becomes easy when you're incrementally breaking it down, reordering it, revising it, etc.