I write for a living. Mornings, I work on my fiction--right now, a YA novel about teens discovering the existence of alternate realities. The rest of the day, I write marketing pieces. For now, I have a steady contract with a place that provides information on the medical device market.
A writer can get writer's block. A marketing writer cannot get marketer's block. Sure, a day or so might go by when you have trouble organizing your thoughts and tasks, but it better not go on much longer than that. Because your clients kinda notice that. You have press releases every week, pieces to go out, edits to respond to....
It's useful to take the mindset and move it to the fiction. It's a job, after all. A job I chose, a job no one needs for me to do. But a job.
So, if you're a writer and wondering whether a day job writing would use up fiction resources, don't worry. It may take some energy, but the skills you learn can be immensely useful.