Workouts in the 5k Training app are based on time so all of the interval training sections can be included within a workout. Basing them on distance would make it possible to miss some exercises or sections of the story, and we would also not be able to offer an alternative tracking mode to GPS.
The exception to this is the final 5k run. To complete this run you must complete 5k, and listen to at least 51 minutes of the final workout. If you have not listened to at least 51 minutes of the final workout then you will find that it won't be marked as completed, regardless of whether you reached 5 kilometres within your run or not. However, you do not have to complete the 5k within 51 minutes.