Of course it's extremely likely we'll win either way. But with the runs flowing as they were 750ish was easily achievable. If the SA openers play themselves in today on what looks a cracking pitch for batsmen it's not inconceivable they could avoid the follow on.
To be honest I would have kept batting. Admittedly SA look sloppy and shattered but if we'd reached 700-750 they would have been completely demoralized.