Negative splitting is the ideal way to run a long distance race such as a half or full marathon. Many runners do the opposite by going out too fast in the beginning, and then slowing down significantly in the second half of the race. It's a common mistake because you feel rested and strong in the beginning, so it's tempting to go out fast. It takes a lot of discipline and practice to achieve a negative split. Most people can't do it in their first marathon . But, generally, if you can hold back and conserve your energy in the first half of the race so that you can run faster in the second half, you'll perform much better overall.
Also see: How to Run a Faster Marathon
How to Avoid Hitting the Wall
Marathon Race Day Advice