That sounds like a very productive way to write. I think I'm jealous.

My first drafts are almost completely opposite from yours. I start with a nebulous theme or idea and work from there. Characters and plot develop (or not) somewhere after the fifth page or so. I write slowly, edit obsessively as I go, and change the phrasing of almost every sentence at least two or three times. It's also preposterously long. I'll write three pages and only end up keeping one sentence. A 10% loss would be wonderful; I'm lucky if I keep that much.

Ah well, I guess every person's brain works differently.