I can't say I've followed the Daniels plan training for a marathon. Hopefully, someone can weigh in on this specific training plan. However, I will say this. I question myself a lot when doing grueling workouts. To me, it just means I'm training hard and I'm doing what I need to do in order to accomplish an aggressive goal. Some workouts are supposed to be tough. Add them on top of day to day training and they can seem brutal at the time but those are the workouts that will get you to your goal.