When you first start doubling, expect to feel a little more tired on most runs, especially on double days. But if you add them gradually and wisely, soon you’ll notice that you recover quicker from all your runs, even on days you don’t double. Your body gets used to running again 6 or 8 hours after a run instead of 24. Your “recoverability,” for lack of a better word, improves. In turn, that should allow you to train harder even on days when you don’t double—with better recoverability, you can push hard one day and not be as trashed the next day.