You're cruising. You've just passed the 14-mile mark, and you're already planning your next race on the basis that the one you're doing right now is (almost literally) 'a walk in the park'. And then you start to feel a little sluggish... You're legs are that tiny bit heavier, that chauffage you thought you'd left behind in your training days has made a timely reappearance and all of a sudden it seems to take forever for the next mile marker to arrive. As you close your eyes and focus on each and every step.
As the pain slowly subsides, and you're greeted with a new wave of spectators getting right behind your every step, you begin to feel invincible again; you're out of the other side, and it's plain sailing from here! You pick up the pace, and already start daydreaming about your bragging rights once you've smashed your predicted time. The mile-markers are flying-by once more.
WHAM. That'll be The Wall.
The truth is, no matter how many times someone tells you not to get over-confident and pick up the pace while you still have miles to go, it's difficult to describe just how much care you need to take not to push yourself too early. There is rarely going to be just one, short period of difficulty during which you'll need to summon your strength of character to pull you out of the slump and set you back on to a good pace. There can be many ups and downs within a race, and the best way of minimising them, both in number and in impact, is to maintain a steady pace of your own throughout.
Seeing the 'ups' as opportunities to knock a few minutes off of your race time can be hugely detrimental in long-distance running. The more you vary your speed, the greater variation you'll experience in how you feel at different stages of the race. If you're feeling good at any particular point, relish it - enjoy the race, absorb the buzz - take pleasure in thinking through the minor details such as your next water/sports drink station, watching the mile-markers go by or simply focusing on your breathing.
The chances are, you won't be feeling like this for the whole race so you may as well make sensible use of it while the going's good!