Speed is a combination of strength and power.
Strength is the maximum force your muscles could produce. It is developed through hill training but also with some weight training that should be focused on lower body parts.
Power is a neuromuscular capacity depending principally upon the fastness at which muscle fibers can be recruited for force execution, then movement. It is trained through short sprints and track repeats workouts. For example, 60m short sprints repeats with complete recovery is an excellent way to stimulate neuromuscular system in a way it is forced to engage as much force as possible in a very short bout of time.
Less structured types of exercise like fartlek or strides are also utilized to develop speed; these are methods not leading to a high level of tiredness while significantly enhancing speed-oriented capabilities of the runner. It is proven that neuromuscular system has memory, then not stimulating speed oriented components during training is a sure way to not succeed in being fast in the future.
When strength and power enhance, and speed is brought to an acceptable level, the next step is to work at maintaining this speed for a longer period. After all it is the prime principle of long distance running: run fast and long. Speed endurance is the term employed to qualify this capacity; it relies upon the muscles ability to maintain the high and fast force production longer and longer.
Challenges to deal with when training endurance is the production of acid lactic in tissue at a certain speed named Lactate Threshold. The fine art of training consists of elevating the threshold so that good speed is kept on an extended amount of time. Admittedly, lactate threshold pace can be maintained for one hour. It is then just above half-marathon pace for non-elite runners.
To train speed endurance, you should make middle to long length repeats on the track at a pace above lactate threshold. That means for example 800 meters or 1k repeats at 5k race pace. Recovery should be kept short to not let muscles the opportunity to recover completely, and then simulate endurance conditions and race rigor.
Finally, working below lactate threshold allows to make longer repeats length or split tempos for a duration that could extend to one hour at marathon pace. This type of work allows to situate precisely in race conditions and to teach muscles what exactly the speed they should maintain for this determinate duration (sometimes feeling so long during the race) looks like.