The food/fuel/hydration thing is a very individual thing. The best time to figure out what you tolerate well is during your longer training runs. You will really benefit from taking some fuel during the race. I go with a breakfast bar or a bagel 2-3 hours before the race, so I'm not running on empty from the start. Resist the temptation to pass up the water stops early. You will pay for that later.
I absolutely wouldn't change shoes types three weeks out.
I wouldn't change what you've been wearing for your long runs as far as clothing is concerned. Being a bit cooler is always good for me, because it helps with fluid loss.
Finally, you just need to relax and enjoy what will be one of the most memorable experiences of your life. I had the injury paranoia before my first marathon so I appreciate your anxieties. I also trained for my first one on my own. Not having people to talk to about this stuff all during your training is not the ideal way to do this. Find a good running group to train with for your next 10-15 marathons. After you run this one, you'll keep going back for more.
Report back after your race. Good Luck.
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