I definitely struggle to write at length when I'm writing creatively. I usually spend far too much time editing as I write, which slows the writing process down significantly and doesn't really improve the results all that much, because I end up editing everything again anyway.
I was introduced to Write or Die. I have yet to try it, but I think it will be motivating. And until God provides me with a job for the next three months, I may set aside a few minutes each day to write creatively. If it seems interesting, I may report my progress.