I haven't tried writing in short bursts of 15 minutes or so, but now I'm keen to try! I can see that it would require planning.

I write a lot of my stories without much planning ahead, shaping the initial idea as I go. But sometimes I plan the story beforehand, designating headings, and adding short notes on what I want to cover under each of them to form a structure. I agree, writing becomes much easier that way.