iPhone battery is dying – Runtastic Road Bike GPS Review

I started using this app from past 1 week. So far the app tracking is good. When I start my ride at 530 am, the iPhone battery is about 75%. However it is dying in less than 25 minutes. I know there is no other apps are using the battery during that time. How do we improve this situation? Is this known bug? Do I need to upgrade pro version? Please suggest
Review by cnu2018 on Runtastic Road Bike GPS.