WKExtendedRuntimeSession how to use?

I'm also trying to get the wkextendedruntimesession to work well. I need to create a timer function without background audio running that needs to run for 15 min to 2hr sessions. I've found that when extended runtime is configured in Physical Therapy mode, a session.start() will run for 1 hour. The white runner glyph appears, and all works well. But after this one-hour session, I'm not able to invalidate and create a new extended runtime session. For my use case, imagine a workout app where the user is constantly turning on and off timers. Because I don't run audio in the background (I've not experimented with this) it seems extended runtime is my only option.