Good advice for starting a light novel?

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I also noticed that writing seems pretty tiresome and exhausting.

Yeah. Keep plenty of water close at hand and take regular breaks. As I said, I don't know about writing but when I'm either composing or notating music (especially on real paper) it's very easy to go hours and hours (at least for me) without standing up or drinking anything. This can obviously very easily lead to extreme tiredness and headaches. Seems like a complete no-brainer, but you'd be surprised. It can really make you hate projects and will certainly drain you enough so that you won't be able to do any more work that day.