The uC won't, the throttle cable is (was in the past) connected to distributor so more throttle means more advance.Using throttle for advance timing is discussionable but in practice it has proven to work very well, and gives a very responsive engine.
If the uC is to be used, that variable relationship needs to be maintained, or the performance of your engine is going to change. And, probably not for the better.
Aren't all digital ports usable as interrupts? Or do I need to add some extra function to accomplish that?