Everyone who's ever run a race, from a 5k up to a marathon and everything beyond, has heard the same piece of advice time and time again: Do not go out too fast. There's no such thing as "banking time" for later in the race. You will invariably think you can hold the faster pace for longer than you actually can, and then you will fall apart and your life will be unbearably terrible for the last half or third of the race. Every runner's goal should be to run negative splits — to run the second half of the race faster than the first — as it is often said that every major running record has been set in exactly this fashion.
In fact, it's such a key piece of advice that two giants of the running world are teaming up to give you what should be all the encouragement you need to set your mind to going negative. New Balance and Strava have just announced the Back Half Challenge, where, if you successfully run negative splits at any USATF-certified marathon between October 9 and December 4, 2016 (and log it on Strava, of course), New Balance will send you a totally free pair of whatever sneakers you want, which, a) is awesome, and b) should give you some indication of how difficult an accomplishment this is to achieve.
On their company blog, Strava also just published a long and insightful post about why, exactly, negative splits yield faster times, along with some advice on how to guarantee your own.