An Ironman takes around 12 hours, give or take, PBP around 3 days. Unless you injure yourself in the Ironman, by the second day of PBP there should be little residual effect. I have done double centuries and fixed gear 400K/600K on successive weekends, and even if you feel a little tired at the start of the second event, there is little reduction in performance. I have also read that top athletes will race Ironman events two weeks apart, but they will not race marathons two weeks apart. This is because even though Ironman includes a marathon, it is raced at a lower intensity.