Set yourself a word count. It can be a daily number, or it can be a weekly. But match it. Don't go over, and don't go under. When you learn how much you can easily write in a sitting you then be able to tell how much you can do at a time. Personally I started out with 1,000 words a day, then worked myself up to 2,000. Don't back down from the challenge of writing if you want to get it done.