Is something that should be incorporated once a week during the season, starting towards the end of the base phase with longer back to backs sessions.
EXAMPLE
30-40km bike followed by a steady 3-8km run. Then implementing shorter sessions once you start to get into race season. After warming up – 5-8minute on bike @ 80-85% of max followed by a 1km run at the same intensity x 3 with 3-5min active recovery between.