I ususally tell people to do about half of the tasks in each chapter. If there is one you missed that is conspicuoulsy haunting you, go back and do it. I suggest doing the tasks you are most drawn to and the ones you are most repelled by. But the "medium" ones, just leave them lie.
I don't really like to make a pronouncement about how and when to do the tasks-- it's a personal thing. Don't beat yourself up. Just do what you can and move on.