Exactly, in a way this is right: you get fitter and you need to make less of an effort to achieve a task.  That's why people lift heavier over time, it's also why people shoot for target paces and target heart rates. Your body adapts, so you adapt the workout.  What's the problem here? You go to the gym to get fitter, it works. Then you adjust your workouts to either keep improving, or to maintain your fitness level. Now, here's the really complicated part for fats: when you're used to running 26 miles a day and, immediately after, lifting for 6 hours without a break and then you suddenly stop and just become a couch potato: you need to adjust your calorie intake otherwise, yeah, you'll get really fat.