The 75km is a rough estimate. There are so many twists and turns, and logs to hop over, giant boulders to climb on top, plus there’s ~100 ladders over the whole train, some in better shape than others. Your actual traveled distance is closer to 90km.

5-6 days is a reasonable amount if you are not insane. The second time I did it, there was a couple that ran it in two days. Only saw them at the start, but presumably they made. The reason they did 2 days is because you can pay to rent a tent + food about half way through. So they were running with just a two litter bottle of water and presumably some snacks.

But if you want to actually enjoy the views … even 7 days is not too slow