I am training for a marathon next year with a 10k and a half marathon along the way. Yesterday I found my run a real struggle. A couple of days before the same run flew by. I don't think the difference was physical - I think it was all in my head. I was truly amazed at the difference the mind can make on your physical capabilities. So my question is, how do you train your mind for long distance running? Any books, tips or web links would be hugely appreciated.