You got good advice from the others, so I can't add much. As they said, you can request a GAC from Guest Services at any park. Whether you get one is up to the CM and how well they understand your needs. From what I have read, you are going at a slow time for crowds. With that and using FastPass, your waits in line should be short.
Most of the rides are continuously loading, so the line keeps moving along (might be slow moving though). The continually moving lines don't have any places to sit - they are just lines separated by rails or ropes that you continue thru. The lines that you would probably need a GAC for more are the ones for shows. In those, you tend to stand with a big enough "batch" of people until it's time to load the next show into the ride cars or theater. Some of those waiting areas have limited amounts of seating, but whoever gets there first, gets the seats.
There are 2 shows at Epcot that you stand (or lean against lean rails) for, the movies in Canada and China. They are 360 movies where the action takes place all around you. There is also a 360 show in MK called TimeKeeper (which may or may not be open).
If you have any time to shop (LOL, not likely the day before you go), you might want to look for a cane with a fold up seat on it. I've seen people use those in WDW so they have someplace to sit with them if they don't need a wheelchair.