The wandering mind is the resistance against which your focus has to work in order to develop. Just as developing a muscle requires frequent, focused use of that muscle, the mind also needs to be taken to the "gym" for a focussed workout.
When your mind wanders (as it will) catch it in the act and then return your focus to your breathing. By adding more time to the exercise you increase the "weight" of the resistance your mind has to overcome, and the more resistance you overcome, the greater your powers of focused concentration become.
Through practice, however, you will become better at recognising when your thought have run off, and you will improve your ability to return your thoughts to the object of your focus. It is to become aware of your thoughts and strengthen your ability to stay with the object you have chosen to focus on (in this case your breathing. Think of the wandering of your mind as you would the weight on the barbell in the gym.
Don't become despondent when your mind wanders during the exercise. Keep your mind on counting your breaths.
