You are correct. They say to go to Guest Services for your medal. DH went, when he went back to the ship for something (I wanted a picture of the medal on the island), however unfortunately they did not have them ready yet. I think he was probably too early.

They did however deliver them to our room for us, right as we were pulling away from CC, along with a very nice note. They really went out of their way.
I have seen reports here of people getting the medal at the store at the end of the 5K however that was not our experience in February. As always, these things are subject to change.
And there was no pre registration as of February. You just run on your own.