Preferably, You must let three-5h of transit time in Johannesburg, so that you have enough time to catch your connecting flight. Transiting in Johannesburg is very straightforward; simply Stick to the signals for ‘Worldwide Transit’. The flight from Johannesburg to Harare takes about 1h 30m. The flights provided are unrealistic https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/