Since you asked about the 10K and I didn't notice a question about the half (and don't know what races you're signed up for), I want to make sure you know that you don't run through MK with the 10K, so there's no castle.
They have traditionally handed you your specific event medal along with the snack box when you are put on the bus once you're pulled from the course. This could always change, but it's what it has been.
But as MES said, you don't get the challenge medal if this happens on either challenge event. (you get the challenge medal after the Half when you go through the specific line and they check your 10K finish info)
That dark-castle info is so interesting. Meanwhile, I just want a chance to actually jog through the castle, rather than having to walk at a slow slow pace while everyone in front of me STOPS to get a selfie before exiting it.