After five weeks of trekking in Nepal, I went to my closet, pulled out a pair of jeans that I haven't been able to wear for at least 15 years and put them on. They are not even tight on me! I got a LOT of exercise in Nepal. It was up a mountain and down the other side all day long. IT was mostly high altitude walking, and I climbed up to a height of 17,600 feet at the highest point. What's more, the altitude made me lose my appetite. I was quite hungry when I got to a teahouse and the food was quite good, but after a few bites I had trouble eating any more. Most evenings, I just had a bowl of soup for dinner. I could not eat rice, potatoes, pasta, or bread. They almost made me gag. But I did get enough nourishment to keep my energy levels up.
I definitely recommend trekking in Nepal if you want to be in the best shape of your life. But, it's like childbirth: you suffer a lot, but when it's over, you remember it as the best thing that ever happened to you.