Half-Marathon A half-marathon is a 13.1 mile running race. Normally, you start training for a half-marathon either eight or twelve weeks in advance, depending on what level of training a person has. Twelve weeks in advance is usually for people who do not have much of a training base or a beginner runner. If you have a running base, then eight weeks in advance is plentiful. It is important to follow the training plan to the best of your ability because then you will be prepared for the half-marathon.