I have a similar health history. My weight has gone up and down over the years. I finally decided to just ignore the weight, and eat reasonably and exercise every day. I am learning to belly dance with the help of a TV show and DVDs. I am up to at least an hour a day, sometimes two hours.
Although I have some cravings for carbs, I resist it and allow myself one dish of ice cream a day in the evening. It is something to look forward to. I do not recommend allowing yourself to feel "starving", go ahead and eat regular portions of healthy food. Discuss any unusual (for you) food cravings with your doctor, they may be related to meds that you can change, only you and the doc can decide that. I do not believe in "dieting", just find a good healthy way of eating.
Keep up with the walking, even when it hurts. You can break it down to three ten minutes sessions of walking if that hurts less. Exercise for folks with FMS is different than it is for folks without FMS. We have to start slow and increase really slowly. Discuss this with your doctor too. My former doctor did it in minutes, not the length of the walk, he said to work up to at least 30 min. a day on most days. He said it really didn't matter to him if I did it in smaller sessions, several times a day.
I have found that even if exercise seems to cause a slight increase in pain, I do feel better all around than if I don't exercise. Laying still feels good until you move, I am so stiff and sore if I have been watching TV for awhile and then get up and move around. My doctor is willing to increase my pain meds if needed, she says that "no one should decrease their activity due to increased pain". She is a good person and a good doctor.
It is part of FMS that we don't feel refreshed after what seems like a good night's sleep. I haven't felt that way but once in the last 15 years. Just stretch, exercise, and get moving. Do the best you can, that is all you can ask of yourself.
Take care, Annette