Ride Report automatically tracks cycling activity and integrates with HealthKit. The app description says it uses the phone's GPS (sparingly) but it seems to undercount mileage compared to either conventional cyclometers or dedicated GPS.

It says it works best with iPhone 5 or newer. It also says it's compatible with iPad and iPod Touch, but since the Touch doesn't have GPS, I can't imagine the app doing anything useful on it, and likewise for WiFi-only iPads, which lack GPS.