Seconds on that congratulations on deciding to run a marathon!
I would avoid going on a formal 'diet' - i.e., one that has you consuming <1200 calories/day. When you're just starting out with training, you don't want to be battling a lack of energy when you're trying to get out the door.
I think my approach would be to stick to a fairly healthy diet (with the carb/protein/fat proportions cited in the previous post) that had me consuming around 400 calories/day less than what I was using - so when you start running more, your calorie intake should go up, but still maintaining a slight 'deficit' until you're at the appropriate weight. I think this should allow you to take off weight slowly (which is the best way if you want to keep it off) but still have the energy to be enthusiastic about your running.
I think there are a lot of 'calorie calculators' out there to let you know what you're burning, but a general guideline is that an average-sized person will burn 100 calories per mile (running or walking). Adjust upwards if you're taller, downwards if you're shorter.
The only other words of advice that I have is to be sure to incorporate plenty of complex carbohydrates (fruits, vegetables, whole grains) in your diet. I know I tend to get carried away with starchy foods (pasta, bread, etc) - especially when I'm running a lot. These are simple carbs and are more easily converted to fat. Of course you need some starch in your diet - just make sure that your 60% carbs aren't all starch :) When I'm trying to take off a few pounds, I try to limit myself to only one serving of starch per day.
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