Not Enough Training Background
The first and perhaps most obvious reason you might not want to run a marathon is a lack of training or experienced running background. The marathon is an arduous event and requires a dedicated training block of at least two months for serious runners and four months or more for newer runners. Remember, the more time you give yourself to train for your goal race, the better your chances of success.
More importantly, training for a marathon when you don’t have the requisite running background is a surefire way to get injured or find yourself disenchanted with running if you’re new to the sport. In my experience as a coach, I’ve found that beginners need to be able to average at least 40 miles per week for 5-6 weeks to increase the chance that they will have a good race experience. This means that you need to be able to comfortably run 30-35 miles per week before you begin training for a marathon.
Too Many Other Racing Goals
To run to your potential in the marathon, the required training is drastically different than any other commonly run racing distance. Training for the marathon requires a very specific 8-12 week training block, which necessitates a singular focus that is often detrimental to your short-term performance at shorter races like the 10K and half marathon.
The mistake of trying to accomplish too many secondary goals while training for a marathon is one of the biggest mistakes I encounter when working with veteran runners. Here’s a typical conversation I’ve had during a first consultation.
Unlike other events like the 10K, or even the half marathon, training for the marathon necessitates a specific focus on physiological adaptations that aren’t of great importance to shorter races. In the marathon, the primary focus of training is developing your aerobic threshold (the fastest pace you can run while staying aerobic), increasing muscular endurance (how long you can run without your legs falling apart), and fuel efficiency (how efficient you can be at burning fat instead of carbohydrates while running at goal marathon pace). At no other race distance are these three training adaptations so important. Therefore, to train for the marathon correctly, you need to temporarily neglect the specific training demands of shorter events.
Furthermore, to accomplish many of the aforementioned training adaptations, you need to practice running on tired legs or with low energy levels. This philosophy is often called “accumulated fatigue.” Basically, this means that the fatigue from one workout accumulates and transfers to the next so that you’re always starting a workout or a long run a little tired from your previous training.