Shifting diet for a 1/2 marathon. Where do I start??

You don't need a specific diet to run a half marathon (or any distance for that matter). You just need to eat a sufficient amount of calories to give you enough energy to handle the run(s).

Eat whatever works for you.

I would start by figuring out how many calories are needed to do whatever you want to do with your weight, whether it is lose, maintain, or gain. Put your goal into MFP to get your calorie needs. Keep in mind that you'll be eating back your exercise calories too, on top of the calorie goal MFP gives you.

If you're looking to lose weight, you absolutely can do it while training, but it can be a challenge for a couple reasons. First, training in a deficit is going to be a bit more stressful for your body, so your performance may not be as strong as it would otherwise, and second, running might make you hungrier Keep an eye on your calorie intake and what the scale does to make sure that you're fueling according to your goals.

In terms of nutrition, many endurance athletes prefer a somewhat higher carb diet, but it's not required. To me, it's more important to figure out what you need to eat before and during a run. Some people like to run fasted, while others need a snack, and some want a meal a few hours before a run. Personally, I can only run about 5k fasted. Otherwise, I might eat a small high-carb snack (say, 100 calories of dried fruit) before I run. If the weather and my schedule cooperate, then I'd rather have breakfast, wait a few hours, and then run. You have to experiment and find out what works best for your body.

You also have to figure out what you prefer to eat, if anything, during your long runs. Some people don't fuel during a half marathon at all, but I think most people will have something during a run of that length. Gels are popular, but you can also use chews, raisins, even candy if you want that. There are a lot of options. I like chocolate Clif Shots because they taste like hot fudge when they've been warming up in my pocket over the last 10 miles I personally don't like the taste of Gu, but again, some people like it. Try several things on your training runs to see what you like. Make sure you try out your fuel of choice on a training run before race day, just in case it doesn't agree with you.

You should also have a hydration plan. Carry water, map out where fountains are on your route, whatever works best for you. Consider an electrolyte drink instead of plain water if you're sweating a lot. Nuun is popular; Gatorade works too. Tailwind will get you hydration, calories, and electrolytes all in one, if you prefer not to eat solid foods while running.

Some people like caffeine during a run, and some gels, drinks, etc. will give you that. I personally don't use caffeine, but it's an option.

Protein is important for everyone in order to build or preserve muscle mass. You probably do not want to eat a lot of protein before or during a run. It's digested more slowly than carbs are, so it doesn't provide such quick energy, and it might upset your stomach. I've occasionally had fruit with peanut butter before a run, but otherwise I'd save the protein for later.

Other than that, you don't really need a specific half marathon diet. You don't have to eat "leaner" unless you want to lose weight. You don't need to eat massive amounts of protein. Just make sure that your calorie intake is on target, and have a plan for race fuel/hydration.