There is no doubt that a lot of the time you spend writing a dissertation will actually be spent reading, and reading and reading, and reading. You need to read a lot more than will go in your literature review. You are reading to find things out about what others have found, thought and said, but you are also reading to learn. Its only once you have really learned what others have found, thought, and said that you can work out what you have found, and think, so that you can say it in your writing.
Dr Inger Mewburn, from Thesis Whisperer calls this the reading marathon and has some advice for surviving it here.