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So awesome you are getting back to your first 5k since high school. For the week of training I would maintain the same number of runs you would do in a typical week, but just cut back a bit on the total distance. So if you had been running 3x/week I would run 3x the week before the run and cut down the distance by 20-30%. Try and get a rest day or two in before the race or get in an easy walk so you feel fully recovered. This week is great time to focus on some rolling/mobility work so everything is moving well for race day. Trying doing some soft tissue work before bed to help down regulate so you are getting a good nights rest.
In terms of diet, I wouldn’t make any radical changes except perhaps getting some high quality carbohydrates throughout the day a few days before the race. Nothing crazy just some foods your body is used to and are easy to digest like fruits, oatmeal, rice, etc. Hydration will be key, but many diets lack the basic levels of salts and essential minerals needed to help cells retain water. So I would try adding in a pinch of salt to help the body absorb all the water you are drinking. Best of luck on your race!