Is that honestly such a big deal? If it's long term chronicle/campaign than i'm sure you have lots of plot threads hanging around waiting to be woven into a new story. If it's a new chronicle just make a session zero - players will keep talking a make most work for you. And if it's a one-shot.. Well, than you improvise, i guess.
That's a fair point. In general when not prepared in long term campaign, I have enough running stuff to not need to prepare more.
But reading some other forum, it seems that some people are super anxious about prep and need hours of prep for every game night. Sure sometimes, I am on a creative mood and want to do these hours of prep, and come with a twisted plot, some maps and illustrations. But very often, I can just use all the ongoing stuff to improvise.