After you stop training for something, your body first rebuilds because it is expecting to have the stress again. Then, it detrains. There is some residual effect from training. The residual effect means that your body retains some of the training effect even long after training has stopped.
This residual effect varies, depending on the person and depending on the type of training. Here is a chart showing the residual effects of various types of training:
Motor Ability Residual Training Effect (days)
Aerobic endurance 30 + 5
Maximal Strength 30 + 5
Anaerobic Glycolytic Endurance 18 + 4
Strength Endurance 15 + 5
Maximal Speed 5 + 3
Table Adapted from Issurin, V. (2001) http://undergroundathletics.co.uk/wp-content/uploads/2013/03/Vladimir-Issurin-Block-Periodisation.pdf
As you can see from the table, endurance has a longer residual effect than speed. So, if you go more than 5 days without speed training, you can start losing speed. This varies depending on the individual, so some people can lose speed after a week of not training speed, and others will lose speed if they go more than 2 or 3 days without speed training.
Therefore, based on this research, speed training should not be neglected for more than a week, even if you are doing a block that emphasizes something else, such as speed endurance or strength. I know that Issurin feels that you should only train one thing at a time, in short blocks of 2-5 weeks. But if you go for 2-5 weeks without any speed training, you are losing a lot of speed. On the other hand, you can go for 2-5 weeks without losing much endurance if you are doing a speed block.
So, I like to alternate between a month-long speed block, then a month-long block of speed endurance training combined with a couple of speed workouts per week. That way, you always maintain your speed and endurance.
Then, when you need to peak, you spend 2-4 weeks doing half the volume, but same intensity, so your body gets super strong and ready to race. And you can do peaks as needed, at varying times during the year- trials, world championships, world cups, etc. Not just one peak, but several peaks since you likely have several big races you need to be in top form for. Just do half the number of runs you would normally do in the week or weeks leading up to a big race.
Some individuals can get by with only one speed workout/week. Others need 2 speed workouts per week. And some need 3 speed workouts per week. It varies, depending on the individual. But going for more than a week without doing any speed training can lead to a loss of speed.
Here is a link to an article about block periodization: http://undergroundathletics.co.uk/wp-content/uploads/2013/03/Vladimir-Issurin-Block-Periodisation.pdf
