overweight and run marathon in 2009

[BostonRun2009]

HI ALL!
I am 55 pounds(25kg) overweight. I love to run and my dream always been to run a marathon. At the moment I can run 5k(3.1mile) in 30min.
I would like to run a half marathon on Feb24,2007(Hyannis Marathon, MA-USA), which I have 88 days to prepare.
I don't have much time because I work full-time and have a 6 months baby to mind.
My plan is to run 50min every week day on my lunch time and run longer courses during the weekends.
Is this a plan that can work out? My main problem is that I can't diet. Although, I believe that when I start getting progress on my running I will feel more determined to choose the right food.
Please, advise me!
Thanks