Mel,
Have a look at google maps and you should be able to string yourself a route through the western cape, northern cape and north west province into gauteng (jnb) over two days rather than slogging it up the N1. If you want, why not stick a post out on the
south african think bike forum here? I'm sure there are people on there who could recommend a route. Are you looking at purely road riding or would you be looking at dirt as well? What bikes will you be on?