My sister runs marathons all the time. How in shape are you? If you can run a 5k, then slowly increase how far you run each week. After you are able to run 10 miles or so, try to go for a half marathon. If you are able to get to around 20 miles in training you should be good to go for the event.

Use training shoes and not shoes designed for speed. When training, I recommend finding a place to run where you can leave stuff along the path to make refueling easier. (my sister does this, leaves a cooler at certain points on the trail to grab a quick drink or energy shot. the stash spots also serve as mile markers.) Begin making sure you eat enough carbs and hydrate starting at least 3 days before the race. Be prepared for your legs to be ridiculously sore. You will not be able to walk properly for the rest of the day.

lol hope this helps.