Hmmm... If you are riding in the suburbs, you may want to make your bike more like a road bike. Like 10 said, gets some smoother wheels.
But it sounds like you are itching to get a new bike. If that's true, go to your local LBS and ride a bunch of them until you know what you like.
But if your goal is really weight reduction and improved physical fitness, you're on the right track. Keep on doing what you are doing. Once the heat goes away you'll be able to last longer. Just keep working on the engine. You're doing fine!
Mikey