Writing always takes me longer that I think it will. It doesn't matter what I'm writing: manuscripts, grants, emails, paper reviews, grant reviews, etc. So, I've come up with a few strategies so that I can keep within the writing goals I set for myself.
1) Break your writing project into small goals. For example: abstract, 2 paragraphs of intro, 3 sections of methods, Figures 1-3, 1 section of results, 4 paragraphs of discussion.
2) Use google tasks to keep track of which of your goals you've accomplished. You can tag each task to a day in your google calendar. It feels so good to cross off a task when you've completed it.
3) Set small rewards for when you accomplish these goals (ie 20 min of TV time with kids, collecting flies in the lab, drink a cup of tea, etc).
4) Time yourself to see how long it typically takes you to accomplish a small goal (like writing an abstract). Write this down!
5) Block time off on your calendar to accomplish each goal. Make sure this is according to how much time each has taken you previously, and not according to how much time you think it's going to take you.
6) If you are falling behind in your tasks, don't beat yourself up, don't feel guilty, and don't just keep adding things to your calendar. Re-arrange your 'to-do' priorities so that you can realistically accomplish the next writing goal within a reasonable time frame. Make sure your writing doesn't eat into time that you should be eating, sleeping, working out and socializing - these are really important for your health.