I actually had a way harder time adapting a story vs coming up with my own. I took a premade adventure that didn't really have a "story", just a bunch of quests the PCs could do to level up before fighting the big bad, and tried to make an actual arc out of it. It was really rough, and some of the quests you could tell were reeeally stretching it.

Came up with an idea for a bad guy. Looked up some lore to see where this guy may have come from, what type of being he may be, etc, and from there came up with a pretty cool back story, a couple of assistants/ underlings, and one or two plot hooks. It's nowhere near done but it's already way more interesting with more room to explore than the "easy peasy rearrange stuff" module I attempted.