I realized you asked what the experience was like, and I avoided answering.
It was a fun experience and moved quickly. I plan to do it again. It is what you make of it.
Every mile has some type of entertainment or refreshment which keeps you going. Based on the 'every mile is magic' theme, I covered the mileage easier than expected. It was great to have the characters, bands, performers, clubs, organizations, spectators, and cheer teams pushing us forward. I could feel positive energy in the air.
It's a great way to see WDW in a new way. How many people can really say they were out for a run with 22,000 new friends at 7am and a monorail whizzes past at multiple points?
I decided to make the most of it and get some pictures with the character groups. Now I have pictures with -
Wendy, Peter Pan, Hook and Smee
Cinderella and Prince Charming
Mary Poppins, Burt, and a Penguin
Goofy and Donald playing golf with a Mickey shaped golf cart
Consider staying on site to use Disney transportation.
Afterwards, my daughter was wearing my finishers medal so people would congratulate her as they walked past. It put a smile on her face and we laugh about it.
It is a fun experience and you will have a great time.
Good luck!