The first training that every long distance runner can use, good for every period of preparation, is to sprint from 60 to 100m climbing. Speed is a quality depending of nervous capacity and muscle strenght. Nervous capacity is the capacity of high explosive concentration, thet you need for recruiting the higher number of fibres of a muscle. Strenght of a muscle is the capacity of producing tension, and speed of contraction.
Our muscles don't work like the engine of a car. If you have an engine able to do 5000 revolutions reaching 180 km of speed, when you go at 100 km of speed you use only 3000 revolutions, but the engine works in the same way.
Instead, if we have a muscle made with 100 fibres, we use the most part of the fibres during max. speed, and only a part of these reducing the speed. For example, jogging you can use 20% of your fibres, ALWAYS THE SAME.
So, when you have to use speed, you are not able to use the percentage of fibres normally resting. These fibres are less strong, but also unable to receive in short time the order of the brain.
Running always at slow speed, you de-fuse your nervous system regarding the fibres that you don't use normally.
So, the best way for training not the speed, but the CAPACITY OF NERVOUS SYSTEM, basic for the speed, is to do short efforts at max.intensity, like short sprints uphill. You must interpret this work in explosive way, like a sprinter, not using progressive speed, because the first aim is to develop the capacity of the brain.
Running for a time of 10/15 sec, you cannot do too much lactate. You can use 1min / 1:30 of recovery, so lactic acid can be eliminated almost totally.
But what you have to remember is that THIS IS A TRAINING FOR THE NERVOUS SYSTEM, needing max intensity, so recovery times are not very important.
Too many times, long runners give big importance to recovery times also in type of training not having the goal of improving endurance, but basic nervous and muscular qualities. This is a mistake, when we are speaking about RAPIDITY we are speaking about a quality, when we are speaking about SPEED we are speaking about the USE of that quality. And, for a long runner, we must train the basic quality before the practical expression of this.
So, is not true that long run can reduce speed, and that speed can reduce endurance. Training is what you do, not what you don't do, and you don't improve your speed IF YOU DON'T USE SPEED, at the same way you don't improve your endurance IF YOU DON'T USE ENDURANCE.