Thought you'd never ask. I just finished (a few minutes ago) a typical writer-chore: proofreading page proofs of an omnibus edition that combines two earlier novels. Proofing pages is something every writer has to do at least once per project, and if the project goes into another edition (from hardcover to mass market, from mass market to trade paper, whatever) then you get to do it again. This involved something over 800 pages of text. Errors on page proofs are usually, but not always typographical. In this case, "protege" with acute accents came out as "protEgE" with grave accents. Printers' software doesn't always like submission software. Orphaned close-quotes are fairly common. Paragraphs sometimes run together (confusing to readers if the paragraphs in question are dialogue by different speakers.) Some corrections are simple and quick to mark and fix, and others take a little longer (if the word is sufficiently mangled by the software, what *was* it? These are books I wrote over ten years ago, and I don't instantly remember which of several words seemed right at the time.) If you try to do too much at one sitting, your eyes glaze over and you miss things.
Page proofs always come with deadlines, so it's necessary to drop the current writing project to get them done by their deadline. I alternated for a few days, but then gave up and just worked on the page proofs.
Interleaved with sessions of page proofing, I worked on an introductory paragraph to accompany the anthology story I finished last week (I think it was last week.) The paragraph is giving me as much trouble as the story. I also dealt with correspondence (from agent, from editor, from readers wanting to know things about the books, or just wanting to hear back from a writer), business-related financial stuff (quarterly estimated tax calculations and checking over a royalty report.)
In the past week, something over half my writing time was spent on this kind of chore: necessary, but not productive in the same sense that writing the next chapter is. I had no idea, before I started getting my work published, how much more there is to the writing life than just writing the story itself or maybe the research needed for that book. It's important, when estimating how much you can accomplish in a month or year, to know that these chores will pop up and eat primary writing time. If I were the super-organized sort of person who has workflow charts laid out, I'd definitely mark whenever I know a new edition's coming out..."next page proofs..." "next publicity blurb..." and suchlike. As it is...they land on me without warning and I mutter a little and get it done.