Hi, I'm Male 18, 5'9ish and about 16 Stone, My job requires me to walk 8Hours a day and that is constant walking, Would i need to add extra exercise?

I don't know how to describe my eating habbits, but more or less something like this;

Breakfast : Bowl of Cereal or 2 Toast

Lunch : 1 Sandwhich on half white half brown bread, followed by orange juice and a nutrigrain.

In between Lunch and dinner : Apple or Crisps with more orange juice / flavored water.

Dinner : Regular sized meals, of anything really, chips, chicken, pie etc

I Dont even eat my stated ammount of Fat and calorie intake.

Yet i dont seem to be losing any wait, what extra could i do??

Please help :-(