My father used to run marathons in under 3 hours, so he can be considered a successful amateur runner. I remember the weeks before his races very well. He would eat a lot of carbohydrates and drink water all the time. Especially 2-3 days before the race he walked around the house with 1.5L bottles of water, taking a couple of sips every 15-20 minutes. Take care not to drink shortly before and after meals, and never too much in one go. Water absorption is physiologically limited and drinking more just flushes valuable nutrients from your system.
He often mixed alcohol-free beer with the water, or added some broth.
I personally suggest eating several bananas a day to stock up magnesium and stretching your legs frequently. Proper hydration, salt, magnesium, and stretching are the best preventive measures against cramps during prolonged physical activities. GL