I spend as much--if not more--time writing non-fiction as I do my mystery novels. At present, I write a monthly newspaper column, an online marketing column, my insurance courses, and several blogs.
I've found that writing non-fiction has greatly improved my fiction. Why? Well, first of all, when you have an entire story to tell in 500 words or less, you learn the importance of words. When I first started writing fiction, my books were LONG! Every edit that involved cutting a sentence, a paragraph, or (God forbid) a scene, broke my heart. Usually, these edits were initiated by my critique partner. Now I do it myself.
Then there's the internal editor. I can turn it off. When writing non-fiction, I churn out the piece quickly, allowing my unconscious to do its thing. Then I flip the editor switch on. Lawrence Block gives just that advice in his Telling Lies for Fun and Profit: write the first draft straight through and do the editing when it's finished. That's one of the best pieces of writing advice I ever followed. Of course, it's a lot easier to do with a 500 or 1,000-word article or newspaper column. But once you get in the habit...
Another item I was able to transfer from writing non-fiction to fiction was my Voice. When writing a column or article (usually in first or second person), it's so very easy to allow myself to come through in my writing. Of course, maybe you don't like the me that comes through--but I do, and so do one or two other people. I love to write in first person and, I believe, am a stronger writer when doing so than when I write in third person. I have seen my third person POVs grow stronger in recent years, however.
Finally, my non-fiction projects force me to write every single day. Perhaps I'm not writing more than a page or two a day on my current mystery, but I am writing.
Which is what we writers do, isn't it?