Preparation for natural delivery

My EDD is coming soon, I always tot I will end up going for c-sec but so far the assessment appear ok for natural delivery and my gynae encouraged it since its my first baby. I am now keen too but I am nervous as I have not been deliberately exercising in the third trimester as it is just too heavy and physically taxing to move around, plus breathless. I read about all the squats, lunges, birth ball twisting, breathing etc that we need to do. Hence I am not sure if I can survive the natural delivery even if i start now at 35 weeks. For mommies who have done natural delivery before, did you do intensive exercises or things like perineal massages to prepare for natural birth? When is too late to start and how much did you do? Does baby head size also determine the likelihood you will tear ?