Hi Sarah! I'm really sorry you're suffering cramps, probably one of the worst afflictions while running!
I sounds like that was just one time, in one day of your life. It won't happen many times! If it does, go see the doctor! There was a period where I was debilitated by it and I can't recall why but they gave me a blood test to check for causes behind it. But please don't give up, and don't lose patience, it sounds like you've been doing so well, think of the strong points and leave it behind! 
From what I've heard/read cramps is caused by a mixture of stuff like low levels of sodium in the muscles, or low levels of -I think- thiamine? (Either way, it's something that is plentiful in tonic water, which may be worth a try!)
Don't give up, it sounds like you're doing so well! Good luck with the half-marathon! 
A