alternator acting up

[Kneale Brownson]

I'm in Columbus, OH, planning to drive back home tomorrow (Sunday) to Traverse City, MI, but today the voltage meter was pointing to 12 volts intermittently.  It would show its normal operating position some of the time, especially if I wasn't operating any big draw like HVAC, but then would go back to centered on 12.  Seems to "work" best if I avoid using power.
   
  So I presume the alternator is failing.  It's been several years since the last alternator death.
   
  How far can I get on battery power alone if it pukes on my 400 mile run tomorrow if I'm not using anything other than the ignition/fuel pump?