Hi Sarah!
Thank you for bringing your question to the Disney Parks Moms Panel!
Congratulations on going for your first full marathon! You have picked the best place to do it. For some reason, 26.2 miles are just much more magical at Walt Disney World Resort!
There will be
Disney PhotoPass photographers lined up lots of place on Main Street U.S.A., including right in front of Cinderella Castle. You will easily see them as you approach the castle. There are generally three or four photographers and runners wait in line. Those lines move very quickly, though.
I usually stop along Main Street and have Cast Members or spectators take a few pictures of me in front of the castle on my phone. I also stop for the PhotoPass photographers...that's just one shot I don't want to miss or have blurry!
The photographers are clearly marked with bright vests, so don't worry about spotting them!
I hope this helps! Please let me know if you have any other questions and good luck!