Kinda like Killa said; it takes stamina and a creative drive to keep the engine running. More often than not, artists and writers alike crash a few times at some point or another.
That being said, I kinda lived in my own world for quite a long while, so I had a lot to write about back then, but even then that supply ran dry years ago.
I think it's better, though, if you handled a smaller story before taking on something like a time-machine story. Re-iterating scenes with different outcomes and stuff takes a variable amount of effort, depending on what you want to change.