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One mistake new runners often make when paring for a half marathon is thinking that the 12- or 14-week plan takes you from the couch to the finish line. All half marathon training plans that range in length from 10, 14 or 16 weeks assume that you've already built a weekly mileage base of at least 15 to 20 miles. Your longest run should also be at least 5 miles.
Anything less than this weekly mileage or longest run mileage will overwhelm your body's ability to acclimate. If you have a solid base under your feet, then when you start your training, you'll only be acclimating to the demands of the half marathon training workouts.
Twelve weeks is a common length of many half marathon training plans, but a quick Google search will bring up plans that range from 10 to 16 weeks. I prefer to use a longer plan (14 weeks) with my runners. The extra weeks allow for a little wiggle room if a runner gets sick or has slight setback or injury.
If this is your first half marathon, I strongly recommend a plan longer than 10 weeks. This will give you more time to acclimate to the training demands.
Not only do the plans vary in length, they also vary in content (the types of workouts, weekly mileage and the number of times you run each week). Study the various plans carefully before picking one. First, find one that meshes well with your work and family schedule.
If the plan has you running every day and you know that's not going to happen, then that plan is not for you. Second, find a plan that matches your running fitness level. If the first long run in the plan is 8 miles and your current longest run is 4, select a different plan.
Often plans are labeled for Beginner, Intermediate and Advanced or Experienced, but even then, read through the plan carefully and make sure it fits your current running fitness level.
Read 5 more steps here: https://www.active.com/running/articles/7-training-tips-for-your-first-half-marathon?page=2