Your welcome, my mom also does pretty well with walking, but the distances in Disney are unreasonable for an older person to handle. Make her use the
ECV even if you can park it for a while and let her walk around a specific area. You will find you can accomplish a lot more with her that way. Especially going into store, let her park and walk in the store, but moving from one area to another take the ECV. Give her plenty of time to feel comfortable with it, and seriously watch the people around her, they are so blind to ECV's as you said they walk in front and too close to the side wheels with little ones.
Also another tip, carry a cheap poncho just in case you get caught in the rain, and try to avoid large puddles - the motors are not made for going thru water, so if you think it might downpour, just pick a spot and hang out until it stops.
Please feel free to pm me if there is anything else I can help you with or give you anymore advice. But really- enjoy the time with Mom, long after the frustration wears off she will be remembering this trip with you. I was so glad we did the trip.