Our CO at TBS said it best ... "OCS is a sprint, TBS is a marathon". You have to do your best to stay motivated through all the BS. A lot of TBS is about teaching you want NOT to do to your Marines. To teach this, they put you through it. You will sit around in your BOQ room from 1600 until 2100 just waiting to be secured for the night. You wont know why you are waiting and it will piss you off. This is just one of the little lessons they like to pass to you. If you dip now, TBS will double your habit. If you dont yet, I think you should start.
I don't really look back wishing I would have prepared more. Just prepare for many long days. The good thing is that you will have some of the best guys around you going through it all just the same. The hours will drag on, but the weeks fly by (if that makes any sense).
Hope this helps, let me know if you have any specific questions.
s/f