You can create a custom workout by scrolling down to the Custom Workouts section at the bottom of the workout selection screen and clicking the "Create a new workout" button.
From there, you can drag and drop the intervals you want to create your own workout.
You can make any workout you can dream up or remix an existing favorite workout!
The iPhone and the iPad Mini version of Zwift do not have the ability to create custom workouts yet, as the UI is too small. We will be releasing an iPhone-specific version in the future.