That's the easiest route to drive. When in Johannesburg, just find the N1 highway and drive North. You will drive through Polokwane City in Limpopo Province. Keep driving North to Luise Tritchard and through the tunnels to Mesina and you will be at Beit Bridge border of South Africa and Zimbabwe. A kilometer or so from the Zimbabwe side of the border there is a tie to your right. That goes to Harare. If you keep straight it takes you to Bulawayo. Period.