How can I maintain staying in my good weight and avoid growing fatter?

If you're current diet is healthy, you really won't have to worry about gaining weight. If you eat a lot, or even some junk food, then thats what I'd recommend changing in your diet. Especially with the amount of exercise you get , I don't think cutting out half the food you eat is a very good idea. I've learned from experience that this is the best way to maintain weight. I did it, and my weight has stayed constant for about 2 years. By the way, eating slowly, not less, will cause you to fill up on less food (you're brain doesn't know you've eaten anything for about 20 minutes).