Training for a marathon is a long process, it takes a few months. The training plans use different phases to prepare the body for the race. It is good to know what these phases are and what you can achieve by completing them.
There are five phases of the marathon training — these also work for any long-distance race. Most of the training plans you will find are 12-24 weeks long, but they include only the middle three phases, leaving out the first and the last one. To successfully prepare for a marathon you need all the five phases.
Understanding the role of each phase is very helpful to create or modify a training plan to your individual taste. This way you can improve both your weaknesses and strengths. A personalized plan is always better than a general one.
If you want to immerse in the details, read a book about marathon training. The most popular ones were written by Pete Pfitzinger, Jack Daniels and Hal Higdon.