Take it from a local (I am from cape town), JHB has a few thing to do and see but nothing supper remarkable. so NO its not a mistake. from what I could gather you only have 10 days so I would skip JHB. In fact I would start in Cape Town, spend at least 6 days there, from there fly up to JHB and take a air charter to Nelspruit (if cost is not an issue) hire a car and head to Kruger. Just a bit of advice on Kruger, although it has plenty of wildlife, it can be very very difficult to see them due to the size of the reserve. A much more intimate experience can be obtained in the addo elephant park not all that far from PE (Port Elizabeth). If you can provide me with a few more details about your interests and what type of things you like to do I will be happy to provide you with some pointers.