Adding one more thing to this—I know I know, and sry OP—it's that writing a book, as time-consuming and difficult as it is, is only one thing to do with the waking hours of a life. I know a few people who are impressively 24/7 writers (kudos! idgi!) but most writers don't and can't.
Fav example: Raymond Carver got high on writing short fiction because it was more manageable for his life as he worked, went to school, raised a family. He's credited with reviving the genre in the country! You have no idea what's going to happen to you or your work. You might as well make it exist.
I wrote a novel off and on during downtime at my day job! It’s just a fun thing to do that keeps me engaged in times when I’d otherwise be bored and scrolling or something. It doesn’t need to be grandiose it can just be a thing you like doing