As I mentioned here, the Geulah Shleimah is made up of 4 individual Geulot, each a more obvious level of revelation.
As I mentioned here, the Geulah at 72 is not really at 72, but some time before that. It is a sort of worst case scenario's Achishenah. We will have a few "easy" chances to cause Geulah Achishenah between now and then, but regardless, we are guaranteed that even at the very end it will be Achishenah.
The reason for that is hidden in the secret of Kol HaTor and Birkat HaChamah.