You could rent a wheelchair for your mom, if she'd agree to it. I can certainly understand wanting to help her save her energy for the parks, but if she tires that easily, it might be better for her to have some help. If she feels silly being in a wheelchair in the parks, you could just offer to give her a ride to the entrance and back. They would probably hold it for you at guest services or something. Or, if she doesn't feel silly in it, you could talk her into using a wheelchair or an ECV during the whole trip. For some people on these boards, it really saved their trip from disaster to have their parent (or themselves) use an ECV in the parks, to ease strain on painful backs or legs.
Heck, if anybody offered ME a ride back to the hotel in a wheelchair, I'd take it in a second! Especially at the end of the day. I don't know about you guys, but it's not brisk walking, like to and from the hotel, that tires me out and kills my feet. It's the endless strolling and standing that you do all day in the parks.