As soon as you start planning one. Most marathon training plans are about 16-20 weeks long, but this is really just the race specific training.  Preparing to run a marathon starts months or even years before the actual race.  The more preparation you do ahead of your actual training plan, the better you set yourself up for success for your race.

These are the things to work on before you even start your training plan:

1. Base building – A marathon plan increases your mileage both across the week and for your long runs, but ramping up mileage too quickly is the fasted way to get injured. Take the months leading up to your training plan to add mileage to you base. That way, when you jump into your training plan, you’re body will be ready to add intensity through speed work and increase mileage. The stronger your base, the less likely you are to get injured and the easier your training build will be.
2. Strength training– It’s challenging to add strength training when you are also building for a marathon. Beginning a strength training regime well before will allow you to easily continue through your marathon training cycle and go into your training plan stronger and more resilient.
3. Foundational Skills – This is an opportunity to work on things that you don’t get to focus on during marathon season. Do a 5k race build that concentrates on shorter distance speed or integrate regular running form drills into your training. It’s an opportunity to tackle foundational skills.
4. Body composition– This is a touchy subject, but if you are intending on doing any type of body composition, marathon training is not the time to do it. The stress and effort that training requires means that, most likely, you will be eating more to fuel your performance, retaining more water because of the carbs, and need to focus on performance, not anything else.

None of these are requirements to have an effective marathon. However, the more that you can build a solid foundation, the more you will get out of your marathon training plan.