A marathon-training program takes around 10 weeks plus or minus. There are many programs of 8,10,12,16 etc weeks that prepare the body and mind for the races. here is why one needs to start training early
Starting early gives the body time to adjust to the aches that come with exposing the muscles to stress beyond what they are accustomed to and still have enough time after this phase to do intense training
Starting early gives enough time to slowly build up mileage without the risk of injury. There is a relationship between mileage run in a week and the marathon finish time http://www.runnersworld.com/run-the-numbers/runners-with-more-training-miles-finish-marathons-faster.
The higher the mileage(up to a certain level based on the individual) the shorter the time it takes to run a marathon. Building this mileage too fast increases the risk of injury.
Starting early gives enough time for the brain to also condition to the workout routine. When you stick with a routine for more than a few weeks, the brain conditions and working out becomes a habit. It forges new neurological pathways that help muscles repeat movements they’ve performed before. Your body’s ability to remember and repeat movements is called “muscle memory.”
Starting early gives ample time for your training plan (see attached training plans) to progress through phases in order to optimally prepare you for your specific goal race. To maintain peak marathon or half marathon shape year round you have to put in weeks of hard work that plays to your strengths, works on your weaknesses, and improves your specific endurance.
Starting early gives you enough time to taper (Tapering means reducing the volume of your weekly training mileage during the final two to three weeks leading up to the marathon). Tapering corrects the accumulated wear and tear of training, helps in replenishing the glycogen stores in the muscles and liver and bolsters the immune system. It is critical for reaching the start line in peak fitness and maximal energy levels.