A new study has compared the training intensity distributions of world-class Kenyan runners, European-level Spanish runners and national-level Spanish runners (https://doi.org/10.1080/17461391.2019.1694077). While long, easy runs were the staple of all three groups, what marked out the Kenyan runners’ training was a higher proportion of short intense interval training and tempo runs. This fits with the ‘polarized’ approach to training, for which there is growing scientific evidence.
What is polarized training and how should it be performed in practice? Find the answers here: https://www.peakendurancesport.com/endurance-training/training-structure-and-planning/polarised-training-goldilocks-approach-training-intensity-wrong/