Hi Linda! Looks like your training is going well! You are going to do great in Jan.!
Last year was my first full at WDW and I was nervous about the same things as you are.....I ended up wearing a fanny pack that fit some gels, my ipod (just in case), a camera, and a few dollars just in case I needed it. I honestly never even felt like it was there. It fit tightly against my body and didn't bounce up and down. As for carrying water, there is so much on the course that I didn't need it. Gatorade is fine for me, so I just took one cup of water and one of gatorade at each station and I felt like I stayed hydrated throughout the whole race. I didn't want to carry anything extra with me on the course that I didn't have to so I ditched the idea of wearing my fuel belt. I did test out my fanny pack once or twice before the marathon on my training runs.
As for the 2 20 milers, what plan are you using that calls for 2-20 milers in back to back weeks, seems like a lot to me so close to the race. I think one 20 miler will be plenty and as Scott mentioned be sure to get in a good two week taper before the marathon.
You are doing great...keep up the great work!!