another vote for 'get out of the airport'
If you don't have to clear immigration, you're talking about a twelve hour stay at the airport, with no fresh air and very little natural light.
Grab a shower at the Sesfikile arrivals lounge (ZAR 250) and you'll be freshened up by 9am - then either Gautrain to Sandton/Rosebank/Pretoria or get a hire car or Uber to anywhere. It's all cheap!