Understanding 'Cardiorespiratory' Exercise and the Calories Burned

Some people go to the gym and step on a treadmill, ending up burning 754 calories!  While that is fantastic, it is based upon someone who doesn't ever exercise and this is their first time using the treadmill in a long time.  Your body gets used to exercise just like your hands are used to picking up mugs or using a fork.  Your body will not burn the same amount of calories if you do the same workout over and over again.  For example...                                                      

Let's say you decide to run mile a day for 3 weeks (resting Sundays). In the beginning you will burn let's say 100 calories.  By the end of the first week you will lose 10% of the calorie burn (burning only 90 calories).  By the end of the 2nd week of running that mile you will lose 20% of the initial calorie burn (80 calories).  Still good, but not great.  By the end of the third week, you will have run many miles indeed but have lost over 50% of the initial calorie burn (meaning less than 50 calories/run)!  The results will have dwindled.               

Now that example is based off of someone who runs with no variables, like on a treadmill.  My advice would be to run outside, which would be better since there are variables and you could take different routes, stretching your body and its ability to do work and hence burn more calories.  All in all, switching up your cardio routine ('weekly' is a good plan) will help you maintain a high amount of calories burned each workout.