Over time we develop our daily routines. They are created around the places we have to be such as work or school.
In order to stay in shape, you need to have a workout routine.
Starting with a trainer or signing up for a class forces you into a schedule that helps you create and maintain a routine. Let's face it, we never "find time" to work out. You need to make time and make it a part of your schedule.
Researchers say that if you do something for 21 straight days it becomes a habit. Try setting an activity time for 21 straight days and see what happens.