This sounds very close to Maladaptive Daydreaming. There was a time I had to spend a few minutes on a treadmill for health reasons and my mind just wandered off. Then I started building stories inspired by something I used to play with. Later on, I'd be waiting for my ride home and taking the opportunity to pass the time writing a few chapters based on what I daydreamed. Created a whole universe in those months, sounds a bit goofy in retrospective but it was fun, I wish I had the time now to finish what I was writing.