Congrats; sounds like you are off to a great start! The suggestion with the weights and incline is a good one too...I will have to try that. I could definitely use the butt workout.
I think your treadmill will be fine for quite a while, but I did read that your body becomes accustomed to the exercise you do, so if you find yourself in a plateau, that should be one of the first things to change up a little bit. Maybe rotate in a workout DVD or whatever else interests you. At least mix up the speeds and do some interval training when you get used to it...keep it up!!