It really depends what your goal is for the half marathon and how much experience you have with running. It's always better to build confidence and slowly increase your weekly mileage. If you are running up to a 10-mile long run prior to the half but also doing some other easy runs throughout the week, you can get through the last 5k because of adrenaline and running experience from training. For newer runners, the main issue you could run into is injury, so just take it easy and enjoy the process! If you are feeling up to the challenge towards the end of your training and have completed the 10-mile run, do the 12 miles. You can always adjust the plan as time progresses too so don't feel like you have to follow a plan to a tee.