Google presumably bases it calculation on the assumption that you drive at the maximum permitted speed, which in the KNP is 50 kph on tar, and 40 kph on gravel roads. That's completely unrealistic, calculations should be based on an average of 25 kph, or less.
It is actually a very nice drive through a variety of habitats with a very good chance of seeing plenty of game. It will afford you the opportunity to see that the Lowveld is far more diverse than one would think if just visiting Lions Sands and Singita. To maximize the game viewing, I would recommend entering the KNP as soon as the gate opens.
I'm sure that an air transfer can be arranged from any suitable airfield, but note that Singita Lebombo does not have an airstrip of its own. You will be flown to Satara in the KNP, from there is almost 30 kilometers to Singita, depending on which road you take. I suspect the transfers are via the H6 instead of the S101, even though the latter is usually far better for game viewing.
Johan