I just returned home from a month of backpacking and must share something, despite being mostly a lurker.
When I left for backpacking, I was about 150 at 5'8''. Not overweight but not thin either, with a true woman's body. I went backpacking throughout Europe with a backpack that stared out at about 20 pounds and ended up at 37 pounds. It was VERY hard at first at 20, but seemed to get lighter as I went on. I only carried it via transporting from country to country, but sometimes would have to carry it for half a day or so.
I walked about 10-13 miles a day sightseeing with a 10 pound day pack, if not my large one. This is easy in Europe when you don't like the metro or buses. A day started at 8:30 and would end at abut 10:00PM. By the end of my four days in each city, I knew it like a book.
Halfway through I noticed my pants no longer fit and a belt was required five hooks in. Then, I noticed my little pouch was gone. This was a shock because I only had a mirror on the fourth hostel in--I wondered where part of me had went.
By yesterday I was excited, thinking I was in the range of 145 pounds. I fit into the jeans I wore at the end of highschool, and shorts--I looked tone in a bikini. Really excited, I did the measurements: 41-31-41.
I weighed myself: 165 pounds.
I literally gained 15 pounds of muscle. I look in the mirror and can't believe this body is my body. I mean, I move my legs, there's muscles--my arms are muscular. My belly is flat.
I'm sorry, but this has literally never been before! Not even when I was 145 pounds! I've never been this fit.
I call this the backpacking diet.

Hopefully I can keep this. I take muscles and being borderline overweight any day than unfit and lightweight. But I'm so excited to go jean shopping!