The short answer to the question of how long it takes to train for a marathon is “It depends”. It depends on your fitness level at the start of your train-up; it depends on your previous running experience; it depends on your goal for running a marathon. If you plan to just finish, you’ll have a shorter train-up than if you plan to place at the top.
Most people who are contemplating running a full marathon within six months from now are already running on some level before they begin their train-up. If you are already running 15 miles per week, then expect your train-up to take anywhere from 18 to 22 weeks. Your plan should include running at least three times per week in the beginning and up to four times per week as your training progresses all the way up to the end. To build up your fitness level and boost your resistance to injury, also work in some type of cross or strength training 1 to 2 days per week.
Runners who may have previously ran a 5k, 10k, ½ marathon or even a full marathon may be ready to run 26.2 miles in as little as 12 weeks. They already know how to train for a race, so their learning curve is a lot shorter than someone who has never run competitively and they need less time to train for a full marathon.
How long it takes to train for a marathon also depends on between just finishing and finishing toward the top of the list. As you can imagine the train-up routines are much different between the two extremes. If your goal is to just finish by either doing a walk/run or a slow run the whole way, then you should be ready to compete in about 20 weeks providing you start your train-up with a base mileage of 12 to 15 miles per week.
If you plan to finish high on the list of winners, then your marathon training can be ready in about 18 weeks, but the schedule is much more gruelling. To be ready in this amount of time, be prepared to run five days per week and be able to comfortably run 8 miles per day before starting your train-up.
Your runs will vary between long runs once per week at an easy pace, tempo runs at about your 10k pace and race pace runs which are runs at about the same speed as you’ll run in the marathon. Throw in some cross training and interval workouts and you have a plan that will have you ready in 18 weeks.
Training to competitively run any distance requires an extreme commitment in both time, energy and resolve. Not only does it take a lot of physical preparation, it takes a lot of mental preparation too.