Change of Diet - It was not until I started eating Thai street food, that I realized just how unhealthily I'd been eating back in the States. My standard "American college student" diet had waaaay too many empty calories in it. I found that Thai food has fresh ingredients, very little sugar, and an abundance of flavor. I found it very hard to eat vegetables before traveling, but now it's second nature. Meals usually keep me satisfied, sating the urge to incessantly snack throughout the day. Also, a Thai milk tea will always beat out soda!
Heavy Lifting - I thought lugging around a heavy backpack and (even heavier) guitar would be my least favorite part of traveling. However, I've found it to be fantastic exercise, as well as an outlet for building endurance.
Sunlight - Sure, you can get sunlight anywhere, but here in Asia it's ALWAYS sunny. In the States I stayed indoors way to much. Why? Because it was easy to, and I was lazy. Now it's not so easy to hide in the dark. C'mon! Gotta go out into the blazing sun to go see Angkor Wat!
Backpacking is a fun way to stay fit or get into shape. In actuality, there are probably thousands of reasons why. Find out for yourself!