The body seems to lose in the extremities first and the core last. I suspect we have evolved like this because the core is the closest to your vital organs and your body wants to keep energy stores near those organs as long as possible.
Having said that, you should start doing crunches to tighten your abdomen muscles, and if you have a gym on campus, you should start doing weight-resistant exercises, either free weights or weight machines. All the walking will not tone your upper body, but using weight resistance machines will. Also, toning your upper body has been shown to increase metabolism more effectively.
Keep it up - you will get there!