Google Fit works even if you don’t have an active 3G Internet Connection.
You will have to have GPS active all the time.
Start Google Fit, go to Settings, slide to “Google Fit data”.
Enable: Activity detection.
High accuracy mode: disabled.
That’s all, just be sure that the GPS service is active.
You don’t have to start the app or do anything, Google Fit works by itself and will record your walkings or bicycle rides / running.
If you can get a 3G connection on your phone, Google Fit will work much better, meaning that will record your activity much more accurate.