Hey girl,
Considering your past history with eating disorders, and how long you walk everyday, and your desire to have your body at a good weight, I'm thinking you would benefit from weight lifting. It maximizes your exercise time. You increase your muscle mass which increases your metabolism, yet you look thinner. Toned, too. And women don't really bulk up unless they intend to and work for it really hard (and take supplements, usually). I'm not suggesting that you ditch the walking, that's good too. But 1/2 an hour to an hour of weight lifting (reasonably heavy, not light weights) 2-4 times a week can make a BIG difference! And then you can just do 1/2 hour walks on most days. Just thought I would suggest that, if you haven't considered it before.