The start line is waaaay over in the Epcot parking lot, so nowhere near the Epcot resorts. You'd have to walk all the way through Epcot. It's not open in the morning. Even if it were open, you have to have a ticket and a park reservation. And it's a long walk after a marathon. All of the Disney hotels, even the cheap ones, have a lot of very early buses running.
The monorail would be the easiest to walk from, and it is running on marathon morning. But that is still a long walk. The buses will take you right to where they want you to be at the time they want you there.