A cable that long has some capacitance to overcome. You'll need a fast driver, something that for a 20ma/12V current loop interface for example. And you'll need a receiver to buffer the return back to 3V levels.
I doubt an ADC will work. You are looking for edges to detect the return signal, followed by something to distinguish between noise and actual signal return. A cable that long is an excellent antenna so you're going to get burst of noise from passing cell phones, radio transmitters, lightning EMP pulses, etc. Modulate your outgoing signal with PWM to help separate it from random noise.
You'll need a fast timer to capture incoming edges. Since you might see false triggers ideally you'll want to reset the capture input as soon as it arrives.
Jack Peacock
