Julian, I'm doing my best wagging my finger in your face, pretend I'm your mother thing here. Don't be so foolish!!!! It's your heart guy, it has to pump all that extra slosh around and it's just a muscle, it's not elasticized. Now that the lecture is over, don't eat anything even a little bit salty and don't drink anything during your run tomorrow. Ask if you can run longer (they say here that taking off more than a kilo per hour is way hard on the body, especially the heart) even 5 hours and if you can't, or can't get it all off then see if you can get in another run the next day.
Good Luck and please forgive my lecture, I know it's tough but don't make it worse.