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 final 5k run lasts for 51 minutes. 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.
If you stopped the final workout before the end and don't want to re-do the whole thing you can just resume from where you left off and listen to the final few clips. The workout will then be marked as completed.