Probably my reply is not fully connected with your question, but I want to try to explain why, training specific endurance, you can improve in your speed. The first question is : what the speed is ? Is your max speed (for example, 11.8 for 100m for Gebre) or is "relative max speed" connected with your event ?
Personally I think that speed is every thing faster of 10% of the event speed. So, if you are a runner of 800m in 1:44, your race speed is 13.0 every 100m. 10% of 13.0 is 1.3, 13.0 - 1.3 = 11.7. When you run in 11.7 you go for SPEED, and the fact that you can run 10.5 or 11.0 is not important, because important is to run at 12.5 speed longer possible. Of course, the same speed has different goals in relation with the distance. 11.7 for 100m is speed, 11.7 x 3 = 35.1 for 300m is speed endurance, 11.7 x 4 = 46.8 for 400m is the BASIC SPEED for the final distance.
For a Marathon runner of 2:06 (3:00 per km), 110% of the speed is 2:42 per km. Of sure Baldini never run faster than 2:42, but about 3:00 he tries to develop his SPECIFIC ENDURANCE. For example, starting with 4 x 5000m in 15:00 rec. 1000m in 3:30, the final workout can be 4 x 5000 always in 15:00, but recovering 1000m in 3:10, eventually adding a final 2000m in 5:45. THIS IS A DEVELOPMENT OF SPECIFIC ENDURANCE.
Now, I want to give an example.
You run 10 times 400m in 60.0, recovering 1 minute, reaching a final level of lactate of 12 mmol.
After a mix of different type of training, 2 months later, you become able to run not 10 times, but 12 times in 60.0, reaching the same level of lactate (12 mmol). Do you think that, when you run 12 times, after 10 times the level of lactate is already 12 mmol ? Of course not, may be about 10. But, if you want to go for ONLY 10 TIMES at a final level of 12 mmol, you can run no more in 60.0, but in 59 or less. SO, TRAINING SPECIFIC ENDURANCE AT A SPEED THAT IS NOT MAXIMAL, YOU CAN IMPROVE YOUR RELATIVE SPEED.
But, if you are able running 1500m (that is a short distance) in 3:43 when you run 10 times in 60.0, you become able running in 3:40 or less when you are able running in 59. AND THIS WITHOUT USING MAX SPEED.
Of course, you must use continuous run at high intensity for 20-30 min, and also short sprints for training your neuromuscolar system (the answer of BONO is very correct).
This is the reason because, in the modern training, long run at low intensity is useless, and max lactic speed also. And this is the reason because I use so much hills of different length at max intensity.