True, simple carbs like sugar and highly processed white flour are going to give you energy highs and lows and cravings.
Complex carbs as BD says are better and digested slower. (If your bread is the pulpy white stuff, it's doing very little for you energy or nutrition wise - try to wean yourself off it and onto multigrain, wheat or wholegrain stuff that fills you up and tastes rotten with vegemite!)
Make sure you eat a good breakfast within an hour of waking up - complex carbs, mainly unsaturated fats and lean protein. Have a fruit or protein-based snack within 3 hours of breakfast and a decent lunch, then another healthy snack mid afternoon and a small evening meal. Eating small(ish) regular meals throughout the day will keep you fuller longer and reduce your cravings.
You need to break the habit of getting home and eating vegemite toast, maybe by getting home, closing your eyes as you go past the kitchen and getting your walking shoes on. Leave the house and distract yourself with some exercise till the craving passes.