You can always break big tasks down into smaller parts. And sometimes it helps to do so in order to stay motivated. For instance, you could say I am going to write two paragraphs, then I’m going to spend 30 mins collecting and organizing source material for the next bit and then I am going to write one page.
I would work on it do 30 minutes and then break for five. Then I would work on it for 20 minutes and break for 10. After that, I will work for 10 minutes and break for 20. Then I will go back to working for 30 minutes and continue the cycle.