The types of workouts and when to do them. Simple as that. How much mileage you do in marathon training depends on how experienced a runner you are and how much time you have to train. In the 12-week schedule I've provided, I recommend that you plan to hit a peak mileage of at least 50 miles. This does not mean you'll run 50 miles every week.
Weekly mileage varies.
Experienced runners or those who have plenty of time to train can, of course, do more. It's up to you to decide what's right for you. Whatever you decide will be your peak, you need to have built up to that level of mileage before you begin the 12-week marathon schedule I've designed. To do that, look at your current training and first arrange your workouts in the same pattern as those in the 12-week schedule. In other words, do your long runs on the same days of the week, your easy runs on the same days and so forth.