Another option from the rollator would be to rent a manual wheelchair and push it when walking. The seat will be far more comfortable and have more back support. Also, I don't know if your husband would be able to push you through flat, smooth queues, but it would give you that option. If you rent a wheelchair (or rollator) from an outside vendor, then you'll have it for waiting in bus queues and you could consider then taking WDW transportation, especially to the Magic Kingdom where that will get you much closer than driving will (plus it's cheaper).
I was misdiagnosed with ME/CFS (in the US it's called CFS/CFIDS and most people won't have any clue what you mean when you say ME) for a while and was an international member of AYME for a very short time (just to give you a sense of where I'm coming from with my advice). Several of my symptoms are similar to yours. I'd say it really is going to come down to what you mean by "short distances" for walking. If you're constantly limited to the 50m limit that's been around for DLA higher rate mobility, you're going to have problems. There are quite a few rides and shows where the queue is longer than that (probably the majority). The queues are also moving often enough that there isn't really time to sit, settle, get up move, and repeat. A 50m limit is also short enough that you could potentially have problems getting around some of the resorts.
If you do find that you end up going, but not being able to do much outside your resort, there are things to do around the resorts. If you can swim at all, there are pools. Most have good food so you could always go get some food and people watch. The majority of my holidays involve my hanging out in the hotel I'm staying in so I have loads of sympathy. I'm really good at watching tv and eating takeaway in places where most people go to museums, etc. (The holidays where I don't live in my hotel usually involve staying with friends, many of whom I met through AYME, so we just stay in their flats and watch tv and eat takeaway

) If you find that you can get into the parks enough to find a spot to park, WDW definitely is a good people-watching spot.
I know these don't directly relate to your questions, but two tips:
1) Bring your blue badge for parking. I can't remember what the exact deal is, but it will help with parking in a closer part of the car park.
2) The First Aid centers in each park can be used if you need to lie down for a rest and aren't up to (or don't want to) go all the way out to your resort.