[aerobod]

Under normal use the brake discs/rotors will last about 100,000 km and will have worn less than 1mm each side. 1,000km of normal use will remove about 0.01mm of rotor (about one tenth the thickness of a sheet of paper), so if the pitting from the rust is deeper than that, it won't have worn off yet.

Although the procedure is at your own risk, a good brake bedding should remove a few hundredths of a mm of rotor. Personally I brake from 110 to 10 km/h as many times as it takes in quick succession for the brakes to just start to fade (brake smell and pedal just starts to get soft), as previously mentioned, drive around to cool the brakes after such a session (at least 15 mins of highway driving) without allowing the car to come to a complete stop.

It likely rusted and pitted originally just in the outer band due to salty water just on that band when the car was parked. The reason the corrosive mixture is only on the outer band is due to the previously mentioned effect from the spreader spring in the caliper that pushes the outer edges of the pads apart, causing them to drag more on the inner part of the rotor than the outer edge when the brakes are not applied.