I can understand completely where you are coming from. I have just recently(a week and a half ago) got back on my "lifestyle change" and let me tell you the hardest thing is getting going. You sound
exactly like I did, questioning myself just how to get motivated and how to stick with it. What has helped me the most is planning. I make sure that I will have healthy food available and I try to always plan my meals a day ahead. That way I am not stuck without anything for lunch one day and then decide to just screw it and go get a value meal at McDonald's because it is convenient. Plan, plan, plan!
I also recently read in a magazine that one of the top reasons people do not stick with their weightloss plan is because they are trying to change too many things at once. For instance, reducing your calorie intake, increasing exercise, eating more healthy foods, getting a certain percentage of protein/carbs/fat, etc. I believe each and everything one of those is important, however start with just changing two habits. Focus at first on just reducing your calorie intake and increasing exercise. Once you have those habits in gear and under control, incorporate something new to work on . For instance, work on eating less processed foods. Know what I mean?
And lastly, personally I think walking is a wonderful start to your exercise program. I have read also that people that use walking as exercise usually end up being very sucessful because it is something that they can stick to.
Hope this wasn't too long! Good luck, trust me if I can do it anyone can!
