Unlikely you will find a single delay corresponding to TX_INTERVAL. LMIC schedules processes at specific times, and your transmission is only one of them (disclaimer: I have a vague knowledge of its internals).
One way to introduce sleep/delays is to escape from the main execution model. There is some example where delays and sleep are put just after the TX_COMPLETE event, but during sleep also internal time is not updated so some workaround is needed to avoid that transmission is further delayed to satisfy duty cycle limitations (though if you send every hour it might not be problematic).
You may also use an external RTC to switch on/off the board when needed…