You set the days to train and available workout duration for each day when building your plan.
Then for endurance there is dynamic endurance coming in the AI update very soon. You set a max duration and the AI progresses you dynamically while optimizing your overall progress within the overall plan.
Additionally in the AI update you get a 4 week window where all workouts are adjusted live on every calendar change to optimize your training and give you the optimal workouts combined to make you faster throughout the whole plan.