Some people say that the feeling of "oh shit, can I really pull this off" is a demon that plagues us until the finish line. I like to look at it in a different way.
Doing a half marathon is way easier than warding off dementors because you DO you know it was you! I have pictorial evidence. See, you HAVE run 13.1 miles before!!

(Those hands waiving in the air are undoubtedly shooing away dementors)