What you should do is have a passport sized picture of you when you were two stone heavier with a fag hanging out of your mouth and take it with you whenever you go running. Then when you are about to hit the wall and think 'Oh god, I cant go on any further', I want you take out the picture, look at it and think 'Is this who I am? Is this what I want to go back to?'
My god, I should become a personal motivator or something, Im really good at this.