I guess it depends on how you're treating it. If you just want to bimble round to put a tick in a marathon box then that's a very different animal to trying to race it somewhere near your best ability. To do the latter you really have to want it, you have to be hungry for it, because it really is a big committment with no short cuts and it will test your physical and mental abilities right through from the start of the training program to the end of the race (and sometimes beyond). I would think there are very few people who don't have days when dragging themselves out of the door is really hard work, but I would guess it's mostly those that manage it who are the ones that achieve their goal.
So to answer your question, I'd say that you need to think about what you want to achieve and set yourself a target. I always advise people to set something that is challenging but achievable, something that will need you to work hard but that is not so challenging that it actually overwhelms you and demotivates you. You need to be really honest with yourself when you do this because it has to be something that you believe you can achieve. Once you've set that goal then achieving it should be your motivation.
