There are two methods to make your Apple Watch last longer with Stryd.
Disable GPS Path (from the start screen on the watch app)
By default, the heart rate monitor is on constantly during workouts. This has a large impact on battery life. To disable this, go to Settings -> General -> Workout on your watch and enable Power Saving Mode
If your Apple Watch dies during your run, you can still save your data. When you open the Apple Watch app again (and you are near your phone), the run will be sent to your phone. To make sure the run uploads to PowerCenter, you need to open the Stryd iPhone app.