There are probably as many different ways to train for a marathon as there are runners. No one training plan is the "right" plan. High-mileage plans work great for some runners while plans with lower mileage that incorporate some speed workouts work better for others. Group training works for many, while others like to run solo. My philosophy is as long as you're "running smart" meaning you're in tune with your body (physically and mentally) then whatever methodology you choose should get you across that finish line. In working with many different types of runners, I have discovered some basic marathon training principles that will help you cross that finish line strong no matter what approach you take.
1. High Mileage or Quality Run, but not both. Some plans call for high mileage weeks for most of the training. Other plans pull back on the weekday mileage and incorporate what I like to call quality runs (i.e., intervals, hill repeats, tempo runs, etc.). Either approach is valid, but mixing the two can often spell injury for many runners. It's hard for the body to put in high mileage week after week as well as the quality workouts. The majority of the runners I've counseled and/or trained who have been injured during the course of marathon training, have been trying to get the best of both worlds. Pick one method and stick with it.
2. Pull Back and Build. One of the hardest things for established runners to do when starting marathon training is to pull back on their mileage (both total weekly and the long run). If you're already doing 16-mile long runs and logging 40-mile weeks, there's really not much room for you to build and grow. Your body will love you if you ease off the overall mileage and weekly long run mileage allowing you to heal, gradually building endurance, mileage, and pace over the course of the training. Increase your weekly mileage by about 10% each week putting most of the increased mileage in your weekly long run while keeping the weekday mileage about the same each week.
3. Build-Up. Drop-Back. A great way to allow your body healing time is to insert some "drop-back" weeks into your training. Divide your training into thirds. When you reach the end of the first third of your training, drop back your long run by several miles. For example, if your longest run in the first third is 15 miles, then run 10-miles for your long run in the drop-back week. The week following the drop-back week, run 16 mile for your long run and continue to build until the end of the 2nd third. You'll probably have gotten to your 20-miler, so try dropping back to 15 miles for the 2nd drop-back week. The taper (see #10) is the last drop-back. This build-up and drop-back approach is not only a great way for your body to recoup, it's also a great mental break from the hard training. My runners relish the drop-back weeks.
4. REST! Some runners think they're being a whimp if they take a rest day. Or they think that somehow they're going to lose some ground. Nothing could be further from the truth. One or two rest days during your training week is vital and as equally important as your run days. If you don't allow the body some days to rest and repair, you'll risk overtraining and increase your chance of injury.