Too much, too soon is often the cause of many running-related injuries. If you start too soon with high intensity (pace and/or distance) before your training even begins, you're just asking for trouble. You'll either overtrain and risk injury and burnout, or you'll peak too soon. More than likely you'll find it hard to reach or maintain your race pace or other training paces this early in the game, and this can really play with your confidence.
Your official training plan gives you 16 to 18 weeks to reach these paces. You shouldn't be able to be or expect to be dead on with training paces five or six months ahead of your race. But nonetheless, runners will test themselves and often become discouraged. So, just don't go there.
What You Should Do During Base Building for the Marathon
The preseason or base-building period, however, is the perfect time for adding in two weekly upper body, lower body and core muscular endurance workouts. Building full-body muscular endurance will help you maintain good running form, and therefore prolong endurance on those really long runs coming up in your training.
More: 5 Core Exercises That Increase Stability and Running Efficiency
So, when do you start all of this? Take your race date and back up the number of weeks in your training plan. For my runners, that's 18 weeks. For example, if you're running the 2013 Marine Corps Marathon, then your training (if with me) will start the week of June 23. So, the two or so months prior to June 23 will be your preseason or base building time, and the month prior to the base building will be your offseason or recovery time if you ran a spring marathon.
It's fine to mix it up during your preseason or base-building phase with fartleks, tempo runs, hill work etc. But, only one a week (if that) with the purpose of keeping it fresh and fun, not for training purposes. Don't worry about pace or time for these runs—just run them at an increased pace that's enough to break the monotony.
More: 6 Fartlek Workouts for 3 Training Phases
Your goal during the base-building phase isn't to kill yourself in each workout. You're just building miles, letting your body get acclimated to the miles, and building endurance with a moderate mileage base.
The 16 to 18 weeks of your "real" marathon training is the time to discover your abilities. As I always tell my runners, trust in your training, believe in yourself, and conquer your goals.
More: How Much Marathon Training Do You Really Need?
Sign up for your next marathon. 