I agree that "just get out and run" isn't really helpful for training for a marathon. It helps to have a plan and to follow one of the many training plans. For example, you can do 25 mile weeks by running 4 miles a day 7 days a week or by running 3 or 4 days a week with purposefully different workouts (e.g, long run, tempo run, recovery run, etc.) The purposeful workouts are going to give far more bang for your buck.
Also, one long run per week -- building up to 20 miles or so a month before marathon -- is the mainstay of most good marathon training plans. It's foolish to run a marathon without having prepared with some "long runs". And you'd be surprised at how slowly you can run the long runs in order for them to be beneficial. In fact, pushing yourself too hard on them is more likely to do harm than good.
It also helps to look over the various training plans to find out what works, specifically regarding training mileage and training speed. You don't need to do much if any "speed work" for a marathon, certainly not if you're not planning to do at least sub 3:30 or so.
Finally, people should be aware that based on your training runs and a practice 5k,10k, or half-marathon, you should be able to gauge your likely performance in a marathon quite well. This is of paramount importance, because you have to know what you're capable of and pace yourself properly in a marathon. Many first-timers (and many who who have run multiple marathons) run the first 13 miles of marathon way too fast, which is partly understandable because the proper pace feels quite slow and easy in first half of the race. But it has bad consequences. There is a saying that "For each 10 seconds per mile faster than your goal pace during first half of marathon means you'll be running 60 seconds per mile slower than goal pace in second half." Or hit the wall and not finish at all. Even pacing is the key.
Also, one long run per week -- building up to 20 miles or so a month before marathon -- is the mainstay of most good marathon training plans. It's foolish to run a marathon without having prepared with some "long runs". And you'd be surprised at how slowly you can run the long runs in order for them to be beneficial. In fact, pushing yourself too hard on them is more likely to do harm than good.
It also helps to look over the various training plans to find out what works, specifically regarding training mileage and training speed. You don't need to do much if any "speed work" for a marathon, certainly not if you're not planning to do at least sub 3:30 or so.
Finally, people should be aware that based on your training runs and a practice 5k,10k, or half-marathon, you should be able to gauge your likely performance in a marathon quite well. This is of paramount importance, because you have to know what you're capable of and pace yourself properly in a marathon. Many first-timers (and many who who have run multiple marathons) run the first 13 miles of marathon way too fast, which is partly understandable because the proper pace feels quite slow and easy in first half of the race. But it has bad consequences. There is a saying that "For each 10 seconds per mile faster than your goal pace during first half of marathon means you'll be running 60 seconds per mile slower than goal pace in second half." Or hit the wall and not finish at all. Even pacing is the key.