Getting to the top of ND (or at least as high as they'll let you go without accident insurance) is quite a struggle and they don't let you dally there all day, either.  10 minutes (max) and mademoiselle is herding you toward the 'down' staircase.  I asked mademoiselle how often she did the trip and she told me "twice a day: once before lunch, once after".  She had, I would guess, less fat than a bottle of skim milk.  You say your diet's not producing the results you want?  I've got just the answer...

From the top of ND, see the 2nd bridge, the darker one.  That is Pont de Sully.  If you were on that bridge and walked toward the left up R. Henri IV in about 4 blocks you would come to the Bastile (that is, the monument which marks where it once stood).  Paris is very compact.