Tracking mileage adds to cardio and uses exactly the same entry that tracking cardio would do. Tracking mileage is not recommended for stationary bikes.
Because your body is working against the resistance of a magnet, not the force of moving your body and bike weight through physical space, stationary cycling is not the same thing as road cycling.
For improved accuracy, buy and wear a Heart Rate Monitor.
Until then, just use whichever of the two entries is closest to your speed.