They need to improve their endurance. All of you can run much faster for a short distance than you can for a 10k road race. Therefore, you do not really need to improve your speed. The problem is maintaining the higher speed for longer distance. The foundation for doing this is not speed work, but endurance training.
Endurance training is achieved by doing lots of low intensity running.
This is because much of endurance training is optimizing the energy delivery systems to and in your muscles. The key is to adjust your endurance training by slowing down and running farther. By doing this, you will receive more benefits with the same overall effort. The easiest way is to use heart rate. We use heart rate monitors for most of our athletes.