I... Don't really agree with this. You probably shouldn't write in a subject that you don't know much about in the first place.. As in, don't do history if you don't read history, don't care about it, don't know about it. Also, feel free to mix up your plan! Choose the thing you really like about what you're writing, and see what works with that, be you in writing.
And a lot of writing is procrastination. Just keep at it in between the distractions! Success at writing (and anything) comes from a place of health and happiness. You can't be productive or creative if you're miserable, so health and happiness come first! Everything else is secondary.