The training schedule has two aspects - distance training and interval training. For distance training the distance is the important thing. If the dog shows signs of tiring, decrease the speed but maintain the distance. If necessary repeat this week at the recommended speed. For interval training it is the opposite.
Speed is the important thing.
If the dog gets tired. Then continue at the proper speed. Again, it may be necessary to repeat a week. By week 12 the dog can easily pass the endurance test, but give yourself a little cushion in case the schedule is too severe early on. That way if it is necessary to repeat a week, you still have some slack.