My love for Runtastic grows by the day. Not only is the service very useful to track my runs and hikes, but its Android app is almost always on the forefront of the latest Google and Android features and guidelines. Case in point: it was one of the first fitness apps to add support for Android Wear and it just got updated with Google Fit integration.
The first time you launch the app you'll be asked to give it access to your Google account (or you can find the option under Settings, Partner Accounts) so it can view and store activity information, location data, and body sensor data (presumably heart rate stats). This integration should alleviate a bit of Google Fit's current woes of erratic and unreliable stats; if you track your activities with Runtastic, you should be able to see more accurate data in Fit for distance, duration, and calories burned.
Both the free and paid versions of Runtastic have received this update, which also adds a new Statistics section to the app with weekly, monthly, yearly, and overall views.