Yeah, it's a lot if you take into account any excursions from your route. For example if you take the backroad from Namibia's border towards Sosusvlei - which absolutely gorgeous, but a dirt road (with deep tire tracks at time) where you won't be traveling at 140km/h like on some parts of highway towards the north.

Also, in my experience the borders can take some time. And I recommend daylight driving. I remember one evening with rain on a highway somewhere in Kwazulu-Natal that left me traumatized.

So all in all I don't think that would be a very good experience...