I haven’t parked at Epcot before so I can’t answer your question directly, but I would think the difference would be negligible compared to the amount of walking one will do once inside the park. Will your older family members be renting wheelchairs once you reach the entrance? If not, and if the walk in will really make that much of a difference, I encourage you to manage their expectations about how much walking is involved.
I took my grandmother to WDW for the first time a few years ago and one evening she insisted on leaving her wheelchair back at the room and just using her cane. She had trouble getting from the DHS bus stops to the entrance and then was even more miserable once we got into the park so we went only to our dinner reservation at 50s Prime Time (barely into the park) and then she was ready to leave.
I hope someone else is able to give you the comparison you’re looking for, and hope you all have a great time tomorrow!