Same boat
Boe787ing,
I'm in a similar situation, I received my email last Thursday but am unable to register for the online tests until this coming weekend. This is due to the fact that I am overseas with work at the moment and won't be in a position to have either a steady Internet connection or computer with a keyboard or mouse until then.
I plan to register on Friday and start my 72hrs from there as it specifies in the email the 72hrs starts from the registration for the online tests.
The best of luck to you.