Although I’m now the full-time operator of my own book editing and design business,  before that I spent a few decades as a writer for companies and a university. I think my day-job writing has informed and strengthened what I do now as an author, editor, and designer. Maybe you have a similar story in progress.
I started out as a writer of programmed-learning training materials for State Farm Insurance in Bloomington, Illinois—think the driest technical writing ever. I soon transferred to the advertising department where I learned that writing can be fun and that I was good enough to be paid for it. Thank you, day job.
That led to the advertising agency world in Chicago as a copywriter. I eventually became a senior VP and the agency creative director for a mid-sized national agency. Thank you, day job.
Writing advertising—and particularly TV commercials—helped me learn to simplify and condense complex information into easy-to-understand language and visuals that were imminently approachable. More than that, I think, it trained my writer/editor eye to look for the best words to use to deliver maximum clarity, meaning, and emotion. This has benefited not only my writing but my editing, too, where I can help other writers craft the strongest narrative. Thank you, day job.
During that time I became interested in screenwriting and worked on scripts in my spare time, although all that brevity training did have a funny impact on my first screenplay. By page 9 I had told almost half the story. Unfortunately, screenplays need to be about 120 pages. This led to weaning myself from the tight-tight-tight scripting that 30-second commercials call for and the expanded vision demanded by screenwriting—and, ultimately, novels.
A 2-minute animated commercial I did for Cap’n Crunch cereal led to a new kind of day job—it was the key writing sample that got me a position as a scriptwriter and then story editor at a major Los Angeles animation production company. I got to write “funny” all day, every day. Thank you, day job.
I ended up going back into advertising for a time, but that day job was no longer satisfying. After launching the editing side of my career with my blog, Flogging the Quill,  I landed a job as web editor/content provider for a university, and my advertising/marketing chops were a key factor.
Then I was tapped to create a video team at the university and my work became scriptwriting with editing and production thrown in. My advertising day-job experience in producing over 75 commercials had given me the insights and skills I needed. Thank you, day job.
I’ve been lucky to work as a writer for my entire career because it has helped me grow in my work as a novelist and as an editor—and get me jobs! Working on ad design with talented art directors has also informed the book cover design work that I now do. Thank you, day job.
Perhaps the one downside of having a writing/creative day job was that, by the time I got home at night, I’d spent the best of my creative juices and that left little for writing screenplays and novels. But, like you I’m sure, soon enough the urge to create my own stuff led to early morning hours that have produced four novels and a book on writing craft .
Now I’m on my own and having fun editing manuscripts and designing books for clients with skills honed by my day-job work.
But, still, I’d love to be able to devote full time to just writing novels. Ahhh, that would be good.
What about you? Has your day job benefited you in your novel or long-form writing? What are the downsides for you? Did you have to unlearn things taught by your day job? And has your moonlight writing impacted your day job?