I understand how overwhelmed you feel with planning! I have some random tips for you.
1. Wear appropriate footwear and socks. Do NOT wear cotton socks…you will get blisters. I have had very good luck with Thorlo Experia's, as they offer cushioning on the bottom of the sock. I also like Bombas. That being said, also bring some Moleskin (pre-cut it at home with with scissors so you don't have to bring scissors with you) in case you do blister.
2. You will not be able to do everything. You just won't. There's so much to see and do, so don't wear yourself down to exhaustion so that it ruins your vacation. Take time to smell the roses and notice the little things!
3. Get maps as you walk into each park. But also google the maps online if you haven't already, so then you can see how the layout is.
4. Take advantage of the train ride in Magic Kingdom or the Peoplemover Transit Authority ride in Tomorrowland when your feet need a break.
5. Go back to your hotel during the day at some point to take a break and get off your feet. Go for a swim and sit on a lounger in the shade if you can. I always do this, and I take a shower before going back to the park that evening because it freshens me up and makes me feel better and ready to tackle the evening.
6. Sunshine Season's in Epcot (its in the Land Pavilion at Epcot) has the best chocolate chip muffin ever. It's dusted with cocoa powder. You won't be disappointed.
7. While we're on the subject of Epcot, take your time in each "Country"…visit their little shops, and explore the nooks and crannies…the movies that each Country shows are worth taking a look at.
8. Not sure if you're flying, but do the "airline check in" the day before you fly home, IF you can. Not all airlines participate. Ask at the check in desk or google it. I can't remember exactly what it's called, but they will check you in and give you your boarding pass the day before you fly.
9. Have a great trip!!!