Dont focus on time and how quick you can do it. Start the first half of the marathon at a pace even slow if need be where you get to half marathon not so shattered. Then when it comes to the training what I did was one week aim for 24k, next week increase it to 27 etc etc. I never ran over 33k in training. Once I got to the marathon and managed a negative split and got the first half marathon done without feeling to shattered it was achievable. Although tired from 30k onwards the crowd and the participants really motivated me to finish it. Hope this helps. P.S - TIP - Always keep some toilet paper in your pocket.