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Hi there---
I'm no expert but I'll share with you what I've learned from more seasoned athletes and from a book that I recently read (John Bingham's Marathoning for Mortals).
Hydration: Generally you want to consume 8-12 oz. of fluids every 20 mins. Don't wait until you're thirsty to drink as that will be TOO late!!
2) For runs longer than an hour, you'll need to start replenishing sodium, carbohydrates and electrolytes. You can do this either with a sports drink, gel or bar. What you choose often depends on personal preference. Some people have more sensitive stomachs than others and thus stick with sports drinks that are diluted 50/50 with water. Gradually increase the concentration of your sports drink as you get further into your run.
3) Eating on the run, this is again a matter of personal preference but here's what you might want to consider: regarding sports bars such as Clif bars (my personal choice!), be sure to "chase" them with some water so that they digest more easily in your stomach (make sure that you choose a low fiber, protein, fat/hi carbohydrate bar to avoid possible stomach upset.
Gels: I LOATHE them but a lot of people swear by them since you just have to tear open the packet and suck them down.
Regardless of what you choose to consume (i.e. sports drink or bar etc.) I recently read that you should shoot to consume about 100-200 calories per hour.
4) In terms of salty foods, I'm not sure about consuming salty foods the day before. I've heard that you should be particularly sensitive to eating a healthy diet, filled with good carbohydrates like fruit, whole grains etc. the whole week before your marathon. In terms of salt, my friends like to run with a few pretzels. Since you said that you lose a lot of salt when you run, be careful about JUST drinking plain water during your runs. A sports drink will help replenish your sodium levels.
5) The best way to figure out what to choose for your marathon is to practice during your long runs. DON'T try anything new the day of the race.
Hope that helps!! Best of luck on your marathon! You'll have a great time!!
