Congrats, I was pretty much in the same boat, lazy bum to health nut, and I love it, haven't been happier. Though I'm not a runner, I can offer some basic advice on the diet. Eating is luckily very simple, and you wont have to change your diet (assuming you are already sticking to clean foods) Though there are some better equipped users to answer specifics of what calorie intake you should shoot for and what not, as far as foods I would have the following suggestions:
- Eat 6 Meals A day
- Balance Protein, Carbs, and Veggies
- Veggies in two of the six meals
- Use a closed fist or flat palm to measure a portion of carbs, protein, and veggies
- Find a good solid calorie intake based on activity level
- Cheat for one day a week
This is basically from Body-for-life as well as a number of other eating plans. In my opinion after two years of wasting my time with fad diets, this is the best and IMO true way to eat healthy.
The six meals a day will keep your metabolism stoked and will help burn fat quicker, the balanced foods will give your body the nutrients it needs, but not any excess. Also the cheat day will help boost your metabolism by giving your body a shock once a week (opinions vary on this, I completely believe it to be true from personal experience), and it will also fulfill those weekly cravings.
As far as the day of your race, there will likely be some people who well give you specific foods to eat before the race, my recommendation would be to play it safe, and stick with your normal diet before the race, to avoid any unexpected reactions from your body.
Anyways like I said I'm sure there are other users more equipped to help you out, but I thought I would give you some info I saw as most valuable to me in the past.
Best of luck, Brad
My first 15K! 
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