For me, the best way to use Habitica was through a combination of dailies, to-dos, and checklists. If you haven't used Habitica before, dailies are the habits that you have to check off every day or on certain days of the week in order to get your reward; if you don't check them off, you take damage. To-dos don't have a specific due date, so they can theoretically let them sit on your list for years (I have a few untouched to-dos that are nearly as old as my daughter--*cough*updatingbabybooks*cough*), but you don't get rewards from them until you check them off.
When I first started using Habitica, I just had "write" on my dailies. It got me writing that sentence a day, which was an improvement, but I started getting the sense that I could manage more than a page a week.
When you set yourself a weekly quota, use the checklist function so that you can check off each page individually. Habitica gives partial credit for uncompleted checklists, so if you write two pages, you'll take less damage than you would if you hadn't written at all.
(On the left is a picture of the screen where you edit the daily to add the checklist; on the right is a picture of the checklist itself when it's been saved and added to your list of dailies. As you can see, mine is due every Sunday. It's gray on the right-hand picture because I didn't take this screenshot on a Sunday, so the task wasn't due yet.)
One possibility would have been to use the "habits" column. Good habits are things you can do multiple times a day to get a payout, like taking the stairs or eating celery. If you want to, you could make "write an extra page" a habit.
To-do checklists work differently from daily checklists. If I have three of my five pages checked off on the day they're due, I don't get any rewards, but I only take 40% of the damage that I normally would (since I didn't accomplish 40% of my goal). With to-do checklists, though, every item on the list increases your reward; a five-item checklist gives you five times the reward of a standalone item.
I started giving myself a to-do checklist for every ten pages. When I reached page 100, I got to check that off along with all the other pages, then add "page 110" as a new to-do. (Don't ask me why I've apparently written pages 92 and 93 while skipping 91 in this screenshot. Accidental unclicking is a harsh mistress.)
Kablammo--when I complete ten pages, whether it's this week or next week, I get rewarded for each individual page. And that's on top of the reward I get for meeting my weekly quota.
Now that I'm doing revisions instead of my rough draft, I'm going by chapters instead of ten-page chunks; that just makes more sense to me. If you scroll up to my first image of my screen, you'll see that my checklist is for completing chapter twelve, not for reaching a certain page. If I were in my rough draft, of course, I wouldn't know exactly how many pages chapter twelve was going to be, so I wouldn't be able to make a checklist for it.
That's how I used Habitica to go from "write every day" to "write every day and make sure it's a substantial amount on a regular basis." I'd love to hear about other things Habiticans have done!