To complete a marathon is quite an achievement. But it will take considerable time to prepare for one, so please leave yourself plenty of time to do this.
If you would like some assistance in this, I will chat with you to determine your current level of fitness and give you an idea of how long you will need to prepare for it, then I will guide you throughout your training. I will work with you to show you ways to minimize the chance of injury (which is inherant with the volume of training needed to prepare for one). For further information, please contact me.