Fit people burn less calories.

It's true. Bummer, I know. Here's why: You burn the most calories when your heart rate is high; The more you exercise, the more fit your heart gets (remember, it's a muscle!), so it can pump blood more efficiently, and burn less energy (calories) in the process. Get it?

So how do fit people avoid plateau and continue to have fitness improvements?

Cross-training. Doing different things on different days. (Hiring a personal trainer to design a periodized weight training program for you!). Always training harder. Once something gets easy, make it harder. Increase your duration or your intensity. If something isn't very challenging to you, you can't expect it to change your body.