Appetite and hunger are not the same thing. Appetite is the desire for food, and it may have nothing to do with physical hunger at all. Taste, smell, visual presentation, pleasant memories, culture, thought-life, hunger, physiological reactions to particular foods, habits, associations, familiarity, and other factors affect our appetite for various foods. Hunger, on the other hand, is limited to a physical need for refueling.

Two major enemies of healthy eating are:

beginning to eat when not hungry
continuing to eat beyond fullness

Sometimes you have to seize the opportunity to eat when the opportunity is there, like on your work lunch break, even if you know you'd be truly hungry if you waited another hour or so. But whenever you can, allow your true physical hunger to dictate when you will eat and how much. The taste, smell, and feel of food will be so much more sharp and satisfying if you will do this. Stop eating when you are comfortably full and know that you've eaten enough to last you for the next few hours. If you haven't listened to your body's hunger signals in a long time or have no idea how much food your body needs, you will need to experiment and practice for a while.