Depends on the time of year you intend to travel down.
From Johannesburg, you will take the N3 highway down to the midlands and cut through the Drakensburg mountain range. In winter, they can be snowed in, or have heavy mist which can lead to accidents and severe traffic delays. (Remember we do not deal with rain very well, never mind a little fog).............
Otherwise, you can leave the N3 as depicted on the map referenced above, and take the N11 which may be slower due to trucks from the port in Richards Bay cutting across and then joining the N3 to get to Johannesburg.

Whichever route you choose, just take it easy and avoid the minivans and pick-up trucks.
I would actually suggest building in a day as a buffer, both going down, and coming back. Take the scenic route down - 5 hours or so - weather permitting - and then take the N3 back, and spend a night in a resort close to the Drakensburg. The Midlands Meander can actually be quite pleasant for a day or so, then tackle the few hours back to Johannesburg.

On the N3, there are tolls to be paid - I cannot remember if foreign cards are accepted - I know on the route to Maputo, they do not take cards other than SA cards - so, cash only, and tolls are pricey - by our standards.

The highway out of Johannesburg - N3 - is a nightmare - also coming back in. It serves the large inland port, so trucks and containers are a mission on the road, and while well policed in Natal - mainly for speed, the cops do make their presence known closer to JHB since there are many accidents every month involving HDV's - this is also due to the hours that the drivers have to keep awake and the incentives to get to JHB ASAP.

So, think about the time that you want to leave JHB, remember that you are driving (sort of) East - into the sun, so you may delay your departure until, say, 9am and that will give you a chance to minimise the traffic mayhem and have a decent drive down. And, on the return, also remember, that you will be driving into the sun in the afternoon, so again, do not leave too late - from the Midlands, I would say, by 11am at the latest, and again, depending on where you hotel is, you may be going against traffic leaving the city.

As for food offerings on the road - on the N3 there are some service stations with generic fast food outlets. Depending on your needs, I would rather prepare a picnic type of thing before I leave - depending on your hotel placement, stores like Woolworths (NOT like the cheap version found in the UK), or Pick & Pay will have options to make up a decent ration basket and avoid overpriced FF rubbish. In fact, Woolies sells cooler bags at the checkout tills that are cheap enough so you can load up the day before and have enough food for the route down.
I hope that this gives you some sort of guidance - any more advice that I can dispense, please do ask and I will try and assist.