Do You Have the Time?
Training for a marathon takes time and dedication; it's not something you can just jump into and complete in a few weeks. You'll need at least a few months to train for a marathon properly, especially if you have never run one before. Make sure you have enough time during the week to run at least 4 to 5 days per week. Since this is your first marathon, it is recommended that you take off two days per week to let your body recover.
Find a marathon that is three months or more away, depending on your current fitness level. Give yourself enough time to build up slowly and also to account for any (hypothetical) time off for injury, illness or life events that may put a small hiccup in your training.
Set a goal—it can be very motivating. Knowing that you are working toward something can help you focus on your training. Mark your race on your calendar, computer, phone, fridge or wherever you need to help keep you getting out the door.
Build Up Slowly
If you've been running for 4 to 6 months for about 20 total miles per week, you have a good base to build upon for your marathon. Once you start marathon training, you don't want to increase your mileage too soon. Increase your mileage per week by no more than 10 percent to help avoid injury.
Build up for a few weeks (3 to 4 weeks), then take a step back for a recovery week before building up again. When you cut back on your recovery week, you should drop your mileage by about 25 to 35 percent and also ease up on the intensity of your runs. After the recovery week, continue adding about 10 percent per week until your next rest week.
Ease Your Long-Run Pace
When training for your first marathon, focus on maintaining an easy pace on your long runs. You won't need any speed work if you are just trying to complete the race. As you progress through your training, your pace will naturally increase as you build up your miles and increase your fitness level.
Build up your long runs to a total of 20 to 22 miles. You should get at least one long run in, but two would be better. Three? Awesome. Getting in 20+ mile runs will help you prepare for the physical and mental rigors of the marathon. Completing long runs will increase your confidence as well as help you run through difficult times.
Find some target races along the way to help determine your progress. Use each of the target races to test out your marathon pace instead of racing them at the pace you might normally run them.
Let's say you've given yourself four months to build up to a marathon. After one month, run a 10K at marathon pace; after two months, try a half marathon at marathon pace.