I recently lost a lot of weight (around 60-75lbs, hard to tell because it fluctuates and I don't like weighing myself). I eat whatever I want, but what I want is food that will help my body and mind instead of destroying it. So I eat more of that. I still eat junk, I just eat it when I have a craving, which is less often. Before I started losing weight I was eating mostly junk food.
It might help to research WHY individual foods are bad for you. For example, look up the ingredients in a can of coke, then look up their proven effects on the body/mind. Aspartame has been shown to cause cancer in rats and the amount of sugar in one can is terrible for your system. Once you know exactly what that food is going to do to you, you'll be less inclined to put it in your body.
Start putting good things in....herbal tea, lean proteins, vegetables and fruits (I like pomegranates, they are amazing when stoned and when not). A good rule of thumb is to eat anything that could have been made by nature (i.e. not processed). Stick to that rule, and you'll be winning (or losing) in no time. What's the saying....the Lord giveth and the Lord taketh away?
It's a gradual process, just remember that. It took me roughly six months to lose my weight, and I'm still losing.
Also, I can't smoke in my house regularly, so I got into the habit of going out to the forest, getting baked, then going for a nice long walk before I go home. Take an ipod with you if you like, it's not as boring as it sounds (especially if you live downtown

).
Swimming is a great idea. Full body workout, fun, easy, and if done publicly there's sure to be some hot lifeguards.
The following isn't great advice, but I thought I'd lay everything out there. Sometimes I would be given the choice of eating or smoking another bowl, and I'd smoke another bowl. Then right away I'd pick an activity so my mind wouldn't wander to food. Substitute the cravings with things you love and don't plan on giving up. (ever

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I've also been a vegetarian for seven years. But I think I might have lost only 5lbs when I stopped eating meat, so I don't think it's a huge factor.