Dear Giorgio;
Not that I am an expert, but a Marathon as you know requires great skills, endurance and determination to finish but above all, a good preparation is imperative. For a solid preparation you have to run at least 6 days a week 3-4 months prior to the event. I recommend one day easy 10K, one day hill, one day easy 10K, one day speed training, one day easy 10K and then your long run which should start at 10K and then increased every week to reach at least 75% of your planned run event, followed by a well-deserved day of rest to recuperate of course.
Another good idea might be to join a training camp of like-minded people which will undoubtedly motivate you further. A 45 min 10K is a very respectable time, unless of course you are an elite athlete, and will mean an under 4 hour finish, well under 4 hours in fact. The task is to maintain the same pace throughout the race.
This is a great endeavour and while it is very exciting to progress through the training, you must be cautious to avoid injuries which will dampen your spirits and could cost you the race altogether, but with determination, hard work and solitude of many hours you will feel very proud and accomplished to cross the finish line. It's all about completion and not competition!
Good Luck!