Long intervals (three to five minutes) target your cardiovascular system and increase your VO2max (the maximum rate at which your muscles consume oxygen) by increasing your stroke volume (the volume of blood your heart pumps with each beat) and cardiac output (the volume of blood your heart pumps each minute). Research has shown that high-intensity training (95 to 100 percent VO2max) is the optimal stimulus for improving VO2max.
Run long intervals at the speed at which your VO2max occurs (referred to as the "velocity at VO2max," or vVO2max), which is approximately 3K (2-mile) race pace for highly trained runners. If you run 2 miles in longer than about 10 to 12 minutes, however, your vVO2max will be between mile and 2-mile race pace. If using heart rate as a guide, you should come close to reaching your maximum heart rate by the end of each interval. During the final eight weeks of your marathon preparation, do one long interval workout per week.
Most research on endurance athletes has shown that improved performance (from 0.5 to 6 percent) is more likely to occur after a period of tapering. The goal of tapering is to recover from prior training without compromising your previous training adaptations. In other words, you want to decrease fatigue without losing fitness.
You can probably expect to improve your marathon performance by reducing your weekly mileage exponentially for two to three weeks, including interval training (if you've already been doing so pre-taper) to maintain training intensity, and increasing your carbohydrate intake (to at least 70% of total calories) to increase the amount of glycogen stored in your muscles for race day. The exact length of your taper depends on your prior training load, your level of fatigue, and your genetically-predetermined ability to retain your training effects while reducing the training stimulus (i.e., how quickly you lose fitness).
Obviously, if you tend to fall out of shape fast, you don't want a long taper. As you get closer to the marathon, also reduce the volume of intensity by reducing the number of intervals in each session. Although research has shown that reductions in training volume up to 60 to 90 percent can improve performance, the research is limited to much shorter races that are not as endurance-dependent as the marathon. Given the length of the marathon, and thus its large dependence on aerobic capacity, it's probably better not to decrease mileage by as much as 90 percent.
I typically begin cutting my athletes' mileage and the length of their long runs three weeks before the marathon (or up to a week later if they haven't been running high mileage). Reduce peak mileage by 30 percent for the first week, 50 percent for the second week, and 65 percent for the week of the marathon (not counting the marathon itself). Keep the intensity high during the first week, including one long interval workout and one moderately-long run (12 to 15 miles) with slightly less than half at LT pace.
Decrease the intensity slightly during the second week, including two short- to medium-distance runs (5 to 10 miles) at marathon pace. The week of the race, do one interval workout early in the week at either LT pace or slightly faster, cutting back on the pre-taper number of reps. The final week also includes a daily reduction in mileage over the last few days that mirrors the pattern of the weekly reduction. Exactly what you do during your taper will depend on what you did before the taper. While tapering, it's easy to catch a cold, so bolster your immune system by getting enough sleep, taking vitamin C (500 to 1,000 milligrams daily), and staying away from people with colds.
So, if you want to run your best marathon, follow these guidelines. And if you train smart enough, you'll run faster than all of your friends, maybe even fast enough to outrun the elites.