Whatever your reason, hold on to it and remind yourself of it often during the months that lie ahead. 

• Be aware of your limits. Consult with your physician before embarking on any training program.
• Start early: as an aspiring marathoners, run a consistent base mileage for at least a year before embarking on a marathon training program.
• Start small: Running a few shorter races—5Ks, 10Ks, or even a half marathon—is an excellent way to prepare physically and mentally for a first marathon

... is building weekly mileage too soon, too fast.

So don't underestimate the importance of consistently running at least 20–30 miles a week regularly before committing to training for a marathon.

1. Base mileage. Build your weekly mileage over time, running 3-to-5 times per week.
2. The long run. Do a long run every 7–10 days so your body can adjust gradually to long distances.
3. Speed work. Practice intervals and tempo runs to increase your cardio capacity.
4. Rest and recovery. Adequate rest helps prevent injuries and mental burnout.

Nearly all marathons include water and aid stations along the way. But if you plan to carry some of your own water on race day:

• Buy a hydration pack or belt, or carry your own water using with handheld bottles.
• Do long runs on a short loop course, so you can stash water in one spot along the way.
• Plot your long run route to pass water fountains.
• Stash water bottles along your route the night or morning before your run.

For any run over 2 hours, aim to take in about 60 grams of carbohydrates per hour.

As glycogen (primary source of energy during the marathon gets depleted over the course of your marathon, your muscles will begin to tire and feel heavy. While no amount of fuel consumption during the race can entirely replace your depleted glycogen, consuming small amounts of carbohydrates can help prevent you from hitting the dreaded wall.

No new shoes, new shorts, new shirt, new hydration pack/belt or new foods.

Don't drink 3 cups of coffee if you usually have one. Your long training runs are when you should be fine-tuning your clothing, gear and fueling strategies.

• Hydrate well for several days leading up to your marathon. 
• Eat a simple, high-carbohydrate breakfast several hours before the start of the race.
• Lather up with a little Vaseline or BodyGlide in any areas vulnerable to chafing.
• The temperature is apt to rise over the course of the race, so don't overdress.
• If you plan to run with music, check ahead of time whether headphones are allowed on the course; not all marathons permit them.

• Start slowly - starting too fast is a big rookie mistake. 
• Don't blaze by every aid station or try to drink from a cup while running full blast.
• Bathroom lines are longest at the first few aid stations. If you can wait another couple miles without discomfort, it may save you time.
• Enjoy the energy of the spectators.