You’ll come to a point at which you start to tire and feel like there’s not much left in your writing reservoir for the day. This is the time to begin to summarize for yourself where you’ve been, to write down your puzzlements or unanswered questions, to do what Kennet Skier, who taught writing at M.I.T. many years ago, calls “parking on the downhill slope”: sketching out in writing what your next step is likely to be,