A mathematician may try to argue logically that by going out faster in the beginning of your workouts and races you'll build a "cushion" for later. Then, if you start to tire and slow down, you'll still be "ahead." Well, running doesn't work like that. The body breaks down glycogen more efficiently and your muscles work more effectively by doing the reverse. You want to negative split your workouts and races, meaning you get faster as you progress.
Gradually working into a faster pace will not only allow you to run more quickly overall, but you'll also feel much better doing it. Closing fast is much more motivating mentally than getting passed at the end of a race as you pay the price for blitzing out those early miles. Trust me; that cushion theory never works.
Practice PacingYour training build-up for your marathon or half marathon should include a variety of different paces:
short 200 or 400m repeats for bottom speed
- 800m to mile repeats
- tempo runs to practice 5K/10K pace
- longer tempo runs at your half or full marathon race pace
Then of course there are your easy run days when the only goal is to recover. On these days, keep the effort level easy and let that pace be whatever your body needs.
The goal with the workouts listed above is to get your internal pace gauge attuned so you can recognize what these different paces feel like. Even in our day of Garmins and high-tech watches, you still should have a sense of how fast you're running based on effort. This will help you avoid going out too fast in races.