What I mean by this maxim is that you should make an optimistic plan for completing work on your current project. For example, I could probably finish by December of 2011 if everything works out. That optimistic plan is highly motivating.
But if I have unforeseen difficulties and don't meet that deadline, I have still planned for the worst case scenario, which is that is will take me longer. Working steadily but inefficiently, I will still get something done; I will still meet external deadlines (just not my own). Planning for the worst outcome in the first place is counterproductive, however. If my best case date for finishing were 2012, then I might not finish until 2013. My goal now is to see how much gets done by December of 11, but if I don't meet that I will still be in good shape.
You can string together enough "bad" weeks, where you are less productive than you might have been, and still make substantial progress. A bad week, maybe you only had 1-3 days where you made substantial progress, as opposed to 4-7. It doesn't matter. A totally uninspired day of writing still makes its contribution. In fact, I think those bad days are even more significant than the very few days where the muse or duende actually descends upon you.