Your weekly training plan will always include Zone 2 training sessions (they're the ones in green), but why are they included so often?

The aim of training in Zone 2 is to improve your endurance by working at an aerobic intensity i.e. a sustainable pace that you can maintain throughout the duration of the session. You'll notice that your long bike and run sessions are usually set at this pace, for example.

Here's Coach James with a great explanation of the importance of working in Zone 2 and how our bodies need variety in triathlon training.