When you are training to walk a marathon, you need to build up your long-distance mileage slowly but consistently. Start by building up your base mileage to the point where you can walk eight miles comfortably, and are able to walk four miles on three other days per week.
By increasing your longest-distance walk each week, you will build your endurance ability. You stimulate your body to pump fresh blood supply to your muscles, and to build more lean muscle fiber. You also need long distance to toughen your feet to prevent blisters and to practice proper hydration and energy snacking during a long walk. Long walks ranging from 10 to 20 miles also give you a chance to try out your equipment and clothing so you know it will perform during the marathon.
You may vary the exact days of the week, but generally, you should take a day off between each walking day, or take just an easy walk on the off day. This schedule varies mileage-building weeks with base mileage weeks to allow a steady build-up of mileage with less risk of injury.
For those who wish to use 5K or 10K events, such as volkssport walks or charity/fun walks in their schedule: Replace the Saturday 4 miles with the 6 miles (10K) and either reduce the shorter midweek days to 3 miles or, if you tolerate it well, just enjoy the additional two miles. Do not do shorter mileage on your longer day.
Your longest training day should be three to four weeks before the marathon. Then you begin to taper mileage. In the final week before your marathon, walk 2 to 4 miles every other day so you will be fully refreshed and energized before the marathon.
Your long distance training will take you through at least a couple of seasons. You won't be able to avoid different weather conditions, including rain, hot weather, and cold weather.