It’s useful to have experience at the shorter distances before you make this leap. A few 5Ks and 10Ks will give you the experience and confidence to move to the half marathon. How will you know when you are ready? If you’ve followed my 10K plan, you should be good to go, and many of the workouts will be familiar. Otherwise, have a look at the first week of this plan. If the first week looks totally doable, or even easier or shorter than what you’ve been doing, you’re probably ready. If it makes you flinch, start with the 5K or 10K plans.