How do I train for a marathon?

To train for a marathon, start training 16 to 20 weeks before the day of the race. Your weekly milage should already be about 30 miles per week, with a strong 10 miler long run each week.  Gradually build your mileage each week by 10% to 20%.  You can also train based on time on your feet, rather than mileage.  Your diet may need to be adjusted depending on the number of days/week you are training and your basal metabolic rate.  Avoid injury by stretching daily and cross-training at least once a week.  Run at least three 18-20 milers within the last 6 weeks of your training.  Two weeks before the race, taper your runs to be shorter, with your last 2 long runs being about 15-17 miles and 10-12 miles.