When you do the same thing day after day, you get very good at it. in exercise this is called the principle of adaptation. It basically means that we become very efficient by doing the same exercise over and over. this is great for sports performance, but not that great for weight loss, strength increases or physical fitness progression. If you always do the same workout for the same amount of time you will eventually hit that plateau where you fail to see any additional change. one way of overcoming this is to modify your workouts every few weeks and remember the principle of overload.
The principle of overload states that a greater than normal stress or load on the body is required for training adaptation to take place. The body will adapt to this stimulus. once the body has adapted then a different stimulus is required to continue the change. in order for a muscle (including the heart) to increase strength, it must be gradually stressed by working against a load greater than it is used to. to increase endurance, muscles must work for a longer period of time than they are used to. If this stress is removed or decreased there will be a decrease in that particular component of fitness. a normal amount of exercise will maintain the current fitness level signaling that you have hit your plateau.