I did about 500 miles on the AT. I found that my miles per day were dramatically better when I upped my calories per day from 3000 to 4000. But then I wasn't losing any weight (although I was certainly gaining leg muscle mass).

I went back to my pre-hike weight and then some within a few years afterward.

I do think if you've got the time and inclination, a thru-hike is a lovely way to lose weight temporarily!