To race fast, you must train fast. To simulate the first, comfortable half of the race, you won't be able to sustain tempo training levels of intensity for very long (since in a race, discomfort is just around the corner). This is why tempo training occurs in brief intervals followed by short rests. Theoretical comfort levels aside, fatigue in the first half of a race is not so uncommon; it usually means you have not built up an adequate base of tempo ability in your training.
Another way of thinking about tempo training is this: if your tempo intervals are too short, you are probably going to wind up running them faster than race pace, making them speedwork, not tempo training. If they are too long, you are likely to become uncomfortable trying to complete them at race pace, making them endurance repetitions, not tempo training.
Unlike tempo, this refers to the ability to surge or kick at the end of a race (or in instances throughout when a surge is called for). It is not a prolonged effort, but rather a skill available in short bursts when you need it. It entails running faster than race pace. You don't have speed unless you can accelerate above your race pace despite fatigue and when anaerobic acidosis is a major determent in your ability to outpace a close competitor.
Speed is the ability to surge at a critical juncture: While some people make a show of flying at the finish of a race, their speed only indicates that they could have run faster for the entire race. Many runners tack on speedwork at the end of tempo training days, to better simulate the fatigued aspects of the end of a race.
Is there sufficient power in your muscles to run relaxed at your racing pace? Clarke distinguishes comfortable running (tempo) from relaxed running (power). Many athletes and trainers define power as explosiveness--the product of strength and speed--but here we more associate that ability to surge with speed, not power.
Power may be involved in explosiveness, however. The best way to differentiate these two related concepts is to think of tempo as the ability to sustain a specific, objective rate of motion--and power the running economy that allows for this biomechanically with each stride in a relaxed way.
For example, the same two athletes may run a six-minute mile, but the stronger runner will run at a quick, relaxed, steady-state level of exertion, and the weaker runner will keep up only by forcing his pace at the ragged edge of his maximal exertion.
The traditional way to build power is hillwork. This is because the added stress on calves, feet, and Achilles tendons that runners feel when running uphill is the same stress they feel when propelling themselves quickly over flat ground.race.