In a recent Nutrition Notes Daily, published by the American Society for Nutrition, a professor from UCLA School of Medicine was quoted as saying, "it takes a lot of exercise to make up for a little dietary indiscretion."  
A vast body of research supports his assertion. Even so, many of us believe that an extra 30 minute walk will erase the damage of a 500 calorie donut, piece of cake or holiday cookies.  It is not so.  In this past post, I introduced the MET or metabolic equivalent as one way to determine how many calories you might burn doing a certain activity at a certain intensity for a certain amount of time.  For example, I found that I burned less than 500 calories by running at a ten minute per mile pace for just over an hour.
In a more recent post, I talked about the belief we have in the self control of our future self.  For example, the belief that tomorrow you will walk twice as long as usual in order to burn off the extra calories you ate today.  Often tomorrow comes and well, its today, so there you are... pinning your hopes on the next tomorrow. 
This is especially important from Thanksgiving to New Years.  One or two days of indulgence can be self corrected, four to six weeks of eating more calories than you need, will lead to weight gain and extra weight is hard to lose. 
(And when you start seeing weight loss ads in January remember that the only way to lose weight or to maintain a weight that is healthy for you, is to make a change that you can stick with - forever.  A short term plan will get you a short term fix and whatever trick you try will only work as long as you use it.)