The half marathon is a difficult balance between conserving fuel and energy while running just at the edge of your lactate threshold. Start too fast and you'll burn through your carbohydrate stores and bonk. Start too slow and you'll be too far behind in the final miles to record your best time.
You need to find the ideal half marathon pacing strategy that will ensure that you conserve enough energy early to finish strong while still pushing your limits the entire distance.
More: Calculate your Running Pace.
The Overall Strategy
To race your best, you should focus on running a patient and conservative race over the first 3 miles, relaxing during the middle miles, and then attacking the course for the last 2 miles.
Interestingly, every world record from the 1500 meters to the marathon has been set running negative splits—running the first half of the race slightly slower than the second half.
This means that if you want to ensure that you run the fastest time possible, you don't want to run the first mile or two too fast, which is one of the most common mistakes runners make. With the adrenaline and competition, this can be difficult and will require focus. Your normal half marathon pace will feel like you're almost walking, so it's very important you pay strict attention to your pace.
More: Target Pace Training
Pacing Over the First 3 Miles
You should target a pace that is 5 -10 seconds per mile slower than your goal finishing pace for the first two to three miles. While this is a scary proposition for many runners, you will easily make up these seconds by being able to close the last few miles fast as opposed to fading and crawling across the finish line.
Remember that it will feel "slow" and you might be getting passed by people you want to beat. While it is mentally difficult, this is by the most effective way to run a race and you'll tear by those people during the last mile when you're fresh and they are dying.
Pacing for Miles 3 Through 11
At 3 miles, begin to increase your pace and effort so you're running at goal half marathon pace. If you've practiced this pace in training, it should feel like a comfortable rhythm for you.
Be aware that you need to increase your effort to maintain the same pace or run faster as the race goes on. As you get more tired, it gets more difficult to keep running faster, so you have to try harder. Many runners make the mistake of thinking that the same effort at mile three will net them the same pace as it will in mile 11. Unfortunately, with each mile your legs will get more tired and it will get harder to remain on pace. Be conscious of this reality and maintain focus.
After 3 miles or so, start looking around and engage the competitors around you. Find a group that is running your pace or a little faster and latch on. Try to relax and keep your focus on staying with the group, not your splits. Use the group and the people around you to help you relax and take your mind of the distance ahead.