Physiological reaction to going out too fast?

You get tired before the end of the race and slow down.

In short, phosphocreatine depletion will come into play if you go out really fast. At races like these, run around VO2max (1500m = above, 5K = slightly below), you are not able to increase your oxygen uptake any further to replenish the PCr pathway. This will take away your kick at the end of the race, as well as any other ATP generation that you would normally get from the PCr pathway during the rest of the race.

In any race, you recruit muscle fibers in a sequence. There is an optimum sequence for the ideal race rhythm.

Go out too fast and you just don't get the right rhythm, so you have to either slow down and effectively re-start the race or just get progressively slower.

When you get it right, you aren't exhaling too much carbon dioxide, which costs extra energy and tends to get progressively worse. This is wrongly refered to as oxygen debt.

I like to think of the oxygen delivery process. There is a lag effect between the oxygen consumption rate at the muscle and a comparable rate of oxygen delivery to the muscle. So, with an aggressive start, oxygen in the muscle is rapidly depleted, which will steer metabolism towards incomplete glycolysis, which burns up glycogen very rapidly and spits out tons of H+ ions. In essence, things that the brain senses as fatigue and will respond by inhibiting muscle recruitment.

Taking this approach, it is easy to see the value of progressive pacing because this minimizes the difference between oxygen consumption in metabolism and oxygen delivery. A good example, is a well paced progressive tempo run. By the middle to end, it is quite amazing how comfortable running at near race pace is. This may also explain the usefulness of very high heart rates due to adrenaline on the starting line. Just like revving a race car engine.