Pacing is as important to distance running as goggles are to swimming! Running a distance event such as a half marathon is a lot better if run at an even and consistent pace wherever possible.
Your longer training runs are where you will be able to practice running at an even pace. Generally your longest runs of the week are also run slightly slower than your expected race day pace.
Use the first 10 minutes or couple of kilometers to ease into your training pace, and then try and run at an even intensity. You can use different methods to monitor this. GPS watches such as Garmins will give you an accurate readout of time, distance, current pace and average pace. You can also train by RPE – or rate of perceived exertion, where 5 out of 10 would be an easy run, 8 out of 10 would be “hard” and 10 out of 10 would be running from a tiger!