Ma ai or combative distance is probably the most importand component of any martial art - close enough to do but not get done.
Aikido requires a longer ma ai than Judo but how do you maintain that against a guy who's coming at you like a love sick rhino.
Well as you already understand backing up just doesn't cut it - so avoid and enter. Get behind him and you have time to either do something or control the ma ai.
By backing in the face of forward movement up you are allowing the aggressor to control ma ai. Never a good idea.