The finish line has premium access bleachers, (paid ChEAR Squad, I forget what level) but there is plenty of opportunity to stand next to the rails as runners approach the finish line, for no additional cost.
If actually seeing your runner cross the finish line is less important to you, and you have park access for the day, being in world showcase or along the course to spaceship earth is a great place for the marathon.
There is a “Family Reunion” area beyond the finish line where you can all gather - the runner(s) will need to get their medals, go through “the chute” to the gear check tents, and will then emerge into the Wonder parking lot, where you start the morning before heading to the starting line.
If you want the best options to see your runner mid-course, the ChEAR people who travel with me have used the following strategy successfully:
On Half day, arrive at the start line with your runner (bus or drive as you prefer). When you separate from your runner for them to go to the start line, the ChEAR team should head for the EPCOT Monorail station. Ride the Monorail up to the TTC, where you will find the course passing through (4 miles into the race). find a spot along the rail in the parking lot (there will be plenty of space!), and cheer until your runner comes through. At that point, walk back to the TTC proper, past the boat docks, and over to the Polynesian, and out to the road (Floridian way) and find a spot to cheer. This will put you between miles 8-9. When your runner passes, walk back to the TTC and catch the EPCOT Monorail back to EPCOT, and head towards the finish line.
On Marathon day, I am less familiar with the specific logistics, but, they like to plan for Main Street USA (5.5 mi) -> Out front of Animal Kingdom (13.5 mi), In front of Hollywood Studios (22+) and meet at the finish. If your marathoners are slower, it is possible to make a stop inside of EPCOT beneath Spaceship Earth prior to your runner arriving, if they are faster, you’ll probably miss them.
Case in point: the first year I ran the marathon, I was still doing about a 9:00 mile at the end of the race. My sister was outside DHS; she told me as I passed that my friend was beneath SSE (he was literally where he could look straight up to see SSE, as the course goes offstage). As soon as I passed her, my sister caught the bus over to EPCOT (her timing in catching it was fortuitous). As she got off the bus, she texted with my friend to decide whether to come inside - which she did, as I hadn’t passed yet. Upon clearing security and the tapstiles, she met my friend on the way out to head to the finish line - I had just passed.
If you want to “follow” your runner and see them along the course, and you’re taking race transportation, I would be on the bus by 4:30 in order to make the trip. If you’re just meeting people at the finish line, it’s going to be highly dependent on their pace. If you have some idea of their pace, I can give you a better idea what time to try to be there.
Whatever the case, perhaps I’ll see you there - I’m doing the Dopey Challenge that weekend.