How can I increase the time for Apple Watch Mirroring to iPhone.

I want to be able to complete a workout on the stair mill and keep my heart rate in Zone 2. I'm using the Workout app on my Apple Watch. I have been able to do Apple Watch Mirroring to my iPhone, but after about 5 minutes it disconnects automatically and I have to try to screen mirror again. I've set my watch up to keep the screen on continually, but that hasn't fixed the problem. How can I increase the time interval before screen sharing disconnects?