How do I write down and prioritize long term tasks that are due in the future but require work over many days?

You can't go wrong by breaking the task into discrete objectives to be fulfilled a little at a time. I break projects like novels into 1000 word chunks, or articles into 25 minute blocks of research, writing, and editing. I never know how many "sprints" it'll take, but so long as I'm actually working consistently, things inevitably get done.