Not really what you asked, but preparing and improvising aren’t necessarily mutually exclusive.
IE, you can prep some encounters, rooms, vilains etc, but mostly on the surface, they don’t have to be predetermined as, for example, goblins in a cave, or the villain can be anyone or even better, you just improvise him towards the end, for a huge plot twist in your players mind, while you just picked what happened to fit the best at the time combined with what you prepared.
Adapting/reusing on the go what you prepared/envisioned is, imo, one of the best skills a DM can have and work on. And this applies for things you prepared in a previous session but happened to not use at the time.