Exercise is certainly helpful, but regardless of whether your goal is to lose body fat, or put on muscle mass, 70 - 80% of your success will come from your diet, so it is no surprise that you're already seeing results. In fact, the first 20lb or so will be very easy to loose within the first month or two if you stick to what you're doing. However, don't get too cocky with that. Keep in mind that all you're initially loosing is water weight. Very little, if any, body fat has been shed. After the initial couple months or so, the weight loss will become significantly more difficult. This is why you have to get used to eating properly NOW so you can push through those points where it becomes more difficult. But keep it up. Even though you've already started feeling it, you haven't even scratched the surface as to how much better you will feel.
As I said above, regardless of your goals, 70 - 80% of your success comes from what you're eating. In your case, you want to pack in QUALITY calories. You can work out as hard as you like and take every supplement under the sun, but only calories can actually increase muscle mass. Lots of:
Lean proteins - Chicken, fish/tuna, lean steaks, eggs, turkey, peanuts/*natural* peanut butter, etc.
COMPLEX carbs - *Brown* rice, *sweat* potato, whole *GRAIN* (not wheat) pastas/breads/cereals, oatmeal, fresh fruits & vegetables, etc.
GOOD fats - Fish/fish oils, peanuts/*natural* peanut butter, walnuts, almonds, cashews, pecans, etc.
Eat a good portion of food from each category above 5-6 times a day, and watch the difference. Also, make sure you're working out properly, as in doing all the lifts properly, not doing too many exercises per muscle group, not doing too many sets and/or reps per exercise, not training the same muscles more than once, MAYBE twice a week at the most, etc.