A positive frame of mind is important.
I believe that the mind has more power than we think. If you are feeling optmistic and believe you can do it, then your body will follow.
I have just run my first marathon (edinburgh) and i was ok. I told myself that i am going to do it and I am going to enjoy it!
I deleted from my mind concepts like "the wall".......(is only in your mind), and tiredness and all that, and decided the enjoy the scenery, and concentrate in feeling good and a slow steady run/jog,
I made it to the finish line (5:18:43), feeling good.
Of course I was tired, one is human, but nothing mayor,
So i tell myself is all in the mind. If you are positive about it all, you'll enjoy it and you'll do it.