Preparing your mind for marathon training can be as hard as the training itself. It does not have to be that way, though. Certainly, a runner needs a plan if he intends to finish that kind of race, but once the plan is laid out, the only thing a runner needs to focus on is the next day of training, not the entire plan.
Training for a marathon should begin three to six months before the race. For beginners, that time can even be up to a year. The difficulty level of the training should not be such that it is stressful. Contrary to how some people train, it should be mostly easy days interrupted by a few hard days.
Preparing your mind for marathon training is much easier when you understand you do not have to run for two hours per day to prepare. In fact, most days should be short, easy runs. One day a week can be longer, and one day a week should be longer yet.
Running for 30 minutes most days allows the body to recover from the harder run. The harder run should get longer as the race approaches, but the shorter days stay intact. About three weeks before the race, there should only be one more hard run, and then no more. That run should be a 20-miler. After that, resting is the most important thing.
Aside from the physical training, mental imaging is the most important thing. When preparing your mind for marathon training, you need to visualize yourself completing the next training session. You also need to visualize yourself completing the marathon. Meditate on an image in your mind in which you are crossing the finish line. Spend a few minutes each day doing that, and it will happen as you have pictured it.