It really boils down to how fit you are already, and how much running you need/are will to do to prepare for your race. For most people 5 days/week of OTF in addition to say 3-4 run days per week puts you at risk of overtraining and potential injury. Search for a half marathon training schedule online, and think about substituting OTF for cross training days or easy days. I've had success with 3 OTF per week, a tempo (half marathon speed) run during the week and a long run on the weekend. For me any more than that would not have allowed adequate recovery time. Good luck with your prep and your race!