A lot of others would recommend an
ECV or wheel chair, but I know that can be problematic for someone that can walk, but can't stand for long periods of time, I would get a GAC personally, it does not provide for immediate boarding, generally it is up to the CM at each attraction, but if there is a long line they will simply provide an alternative waiting area, in which you can sit, if the line is relatively short they may just get you on the attraction. Again this varies from attraction to attraction and in some cases from CM to CM, also please be aware that where FP is available you may be asked to obtain a FP if the stand-by line is fairly good sized. GACs are there for those with "hidden" disabilities, esp. ones that may not require a wheel chair or ECV. And if there is ever an issue with the standing, even with the GAC talk with the CM, explain nicely that you don't mind waiting, but can't stand for long periods of time I found most of the CMs to be very accommodating, of course I have another issue as well, but I won't get into that. I would also budget for an ECV just in case you find that the walking is too much, as it is a really large place and may be harder to walk it than you think.
I know with me sitting or standing for long periods of time is difficult, so an ECV or wheel chair is not an option, where as with a GAC I generally can sit on a bench where I can get up and move from time to time, but trying to drive an ECV throughout Disney World would totally stress me out as well has have me in mass pain as I would be sitting more than walking, walking does me a lot of good and I have no problems with waiting, even if I have to wait 5 minutes more or so than everyone else, if it means I can be comfortable, in many cases the CMs get you on a bit quicker as they realize that since you do have issues it takes you longer to get from place to place, which it really does for me, but I walk alright just a bit slower than most, but I take the time to enjoy the atmosphere, anyway, my point is that I am more than willing to wait my fair turn, just some CMs choose to let me get on faster and if they do I won't complain about it of course, I just want to be able to not be in excruciating pain while waiting, although I am always in some pain, it can either be less or more.
Ok, sorry for the rant, I just wanted to explain the "rent a wheel chair or ECV" response is not appropriate for everyone, which is why they have GACs.