You could use one of Darrel Taylor's timers to keep track of how much time was spent traveling after each direction command. These direction commands and time values could be stored in eeprom, or an external eeprom, if you needed more space. Since your robot's motors could be slower when the batteries voltage drops, it would probably be better to keep track of distance. But then you would need an odometer of sorts on one of your wheels. You could use a button to start and stop this recording, and a button to start following the stored route.