Marathons Require Recovery
Running a marathon challenges your muscles, tendons, ligaments, and almost every physiological system in the body. It doesn't matter if you crushed your goal or struggled to walk/jog to the finish—26.2 miles is a long way to go, and your body endures tremendous physical duress. Therefore, marathoners need to take downtime after their race.
Most marathoners will swear they don't feel sore three to four days after a marathon. While that may be true, it doesn't mean there isn't still physical damage to be repaired. For example, research shows that two of the best markers of skeletal and myocardial tissue damage, creatine kinase (CK) and myoglobin levels in the blood stream, persist more than seven days post-marathon. While increased CK levels won't cause you to feel sore, they are one of the scientific markers of overtraining.
This means that in order to recover fully after a marathon and ensure that you don't set yourself up for overtraining down the road, you should give yourself two to three weeks of nothing but very easy running.
Why Running Marathons Close Together Limits Your Progress
Of course, you're probably thinking, "Well, if I shorten my recovery time a bit, I can get back to hard training sooner, and turn around for another marathon in 6 to 10 weeks." Sure, you can definitely do this, and I've seen many elite and non-elite runners make it work. However, this strategy only works once, maybe twice in a row, before you start to stagnate.
Let's pretend you schedule two marathons 10 weeks apart. After the first marathon, you take two weeks easy to let your body recover. Then, you factor in at least a two-week taper for the second race. That leaves you a mere six weeks of training. While you can certainly fit some hard long runs and solid workouts into this timeframe, it leaves little growth for long-term development.
Specifically, in six weeks time, you're not able to develop your mitochondria fully. Mitochondria are microscopic organelle found in your cells that contribute to the production of ATP (energy). In the presence of oxygen, mitochondria break down carbohydrates, fat and protein into usable energy. Therefore, the more mitochondria you have, and the greater their density, the more energy you can generate during exercise. Mitochondria density and development peaks at 10 to 12 weeks. Training segments that are less than this length decrease potential long-term gains.
As such, a proper marathon training segment should be at least 12 weeks long. Factor in your recovery from your last race and a taper (which doesn't count as training), and you're looking at 16 weeks between marathons.
Why You Need to Work on Different Energy Systems
However, there is also another factor to consider. In order to make progress from year to year, you must train all of your energy systems, like speed and VO2 max. But, why is this important to a marathon?
In the marathon, the primary focus of training is developing your aerobic threshold, increasing muscular endurance, and improving fuel efficiency. While you may do a little VO2 max and speed training here and there, it's often negligible. As a result, you may go years without improving your VO2 max and running efficiency. In the long term, this will limit your ability to improve at the marathon distance, no matter how many long runs you do.
A good way to visualize this concept is to think of a how window blinds work. To raise a blind, you have to pull two strings at the same time. Each string controls one side of the blind. If we imagine the blinds themselves to be your race performance and the strings to represent separate energy systems, you'll find that you can only raise one side (pull one string) so far before you need to begin pulling the other string. Your body works in much the same way.
As such, repeating a marathon every 16 weeks is certainly not going to give you enough time to train other energy systems like VO2 max and running efficiency, especially if you rehash the same schedule and simply change the paces. This trains your muscles and metabolic systems in the same exact way, which doesn't ignite growth and development.