HI!!! I´m not a B737 pilot, but I think I can help you with the engine run-up procedure.
Periodic run-up must be performed when running the engine at low power (i.e. while taxying), because even though ENGINE A/I is on, the engine is not protected against ice build up on the spinner, fan blades and low compressor inlet stator.
When increasing N1 to 70%, ice is shed before it accumulates to the amount where it can damage the engine if it sheds when the engine is spooled -up for take off.
Thus, if your engine procedure call for 30 min periodic run-up (it seems very very long), this must be performed 30 min after engine start if you are not already airbone (doubtfull)