As a rule of thumb, tracking apps typically use 10-15% of your phone's battery per hour. There are a few things that you can do to help extend your phone's battery life while recording out on the trail:
Turn down your screen's brightness - in addition to turning down your screen's brightness, try to minimize the number of times you look at the screen while recording.
Lock your phone - If you don't need to view your recording screen try locking your phone while recording.
Close apps you aren't using - certain apps will continue to drain your phone's battery even when they are in the background.
Turn on airplane mode - when your phone doesn't have service, it constantly searches to try to establish a connection which kills your battery. In most cases, the GPS capability is still enabled and recording will work just fine. However, some Motorola devices or iOS 8.3 and older disable the GPS in airplane mode so be sure and test this out beforehand.
If you can't or don't want to use airplane mode, you can extend your battery by turning off your Wifi and/or bluetooth, push notifications, and email sync.
Carry a portable charger - Having a portable charger on hand is the best way to ensure you won't run out of battery while out on the trail!
Additionally, you can always check your battery levels by going to settings>battery and your phone will give you a breakdown of how much battery each app is consuming.