Unless you are racing or training at an elite level, the first several miles of your long distance run should be at a slightly slower speed than you anticipate running the last half. It’s best for your body and your mental game to be able to speed up as the run progresses and not to have to slow down because you’ve worn yourself out before the run has been completed. Finishing the run moving faster than you started will help to improve your mental and emotional anticipation of the next long distance run, instead of dreading something you had to push hard just to finish.
Another running tip for long distance is that long distance runners should always stay hydrated, drinking water throughout the day, whether they are running or not, to protect their kidneys and to flush out toxins and waste products. Long distance runners require a good amount of hydration to complete their runs with good performance. The more dehydrated you are when you start your run the more the performance suffers and the more water and fluid will be required during the run.