Yasso 800s
One training tool that you might want to consider when training for a marathon is to incorporate 'Yasso 800s' in your training schedule. The name 'Yasso' comes from Bart Yasso who popularized the concept. Bart has worked for Runners World magazine for many years and has run many marathons and ultra-marathons.
Yasso 800s are basically a form of speed or interval training that is geared to your marathon time goal. The way that they work is that you take your marathon goal time in hours/minutes and convert this to minutes/seconds. For example, if you marathon goal is 3 hours and 40 minutes then convert that to 3 minutes and 40 seconds. Now, run 800 meters (approximately 1/2 mile) in your converted time (3:40 in our example) followed by a recovery period (walk or slow jog) of the same converted time (3:40). Start with three or four repetitions and then try to add one repetition each week until you reach ten repetitions.
Keep in mind that this is merely a guideline. You still need to do your regular marathon training with focus on your weekly long runs to develop your aerobic base. If you can complete the ten Yasso repetitions there is no guarentee that you will finish the marathon in your goal time, however, if you can't complete the ten repetitions then you probably won't finish the marathon in your goal time.