I have witnessed people making the same simple mistakes over and over and ruining their races purely because of poor pace judgement. They have been fit and fully prepared but get suckered in on race day and forget their pace strategy (if they have one), and try to perform well above their capabilities. It is a natural response to competition, it doesn't feel right to let someone swim away from you or bike past, but often this can be detrimental to your personal performance.
In Ironman or half-Ironman competitions, when the gun goes off, there is splashing and arms flailing everywhere, more resembling a 100m swimming race. I guarantee about 90% of the competitors are not finishing the swim at nearly the same pace they started. I have had some of my best swims by approaching them as though I was doing a 1.9km swim TT; although you may lose a little bit of ground at the start, it will benefit you much more over the back portion of the swim. This will also allow you to maintain a good stroke and be constantly in control, rather than trying to swim a 1.7km TT after a 200m sprint.
On the bike, it is also easy to get caught up in trying to keep up with someone who has passed you. This is a good strategy if they are of similar ability, but if they are too quick this can really hinder the back end of your ride, and more importantly, your run. This is where it is useful to have a power meter or heart rate monitor and to learn what your threshold is. The same strategy applies to the run, with GPS these days there is no excuse for running out too fast.
The key for a lot of people to perform to their ability is learning their limits and aiming for an even-paced race; this will result in your optimum performance being achieved, and also make for a much more pleasant race!
By Mike Phillips - Pushys Sponsored Athlete