Eddie Jones was alarmed by the England players' fitness levels at the start of his reign and has been "massively surprised" at how quickly they have stepped it up.
Jones has transformed England since taking over two years ago, winning 24 of his 25 matches at the helm.
The canny Australian oversaw a victory over Scotland in his first game in charge and returns to Murrayfield on Saturday with England on course to become the first team to win three consecutive Six Nations titles outright.
However, Jones questioned what he had taken on during his first training session.
The England head coach said: "After the first training run I was thinking: 'Goodness me, what have I got myself into here?' After 20 minutes they were just shot. They couldn't run any more."
Jones added: "They weren't fit. They wanted to play a system of attack and a system of defence and I thought: 'This is going to be hard work.' I was massively surprised how quickly they changed.
"It is hard for good players to change and it is a great credit to them that they have been able to accept they needed to.
"The basis of Test match rugby is physical condition. You've got to be fit enough to play. And when you win games, as we have consistently in the last 20 minutes, part of it is fitness and part of it is tactical nous.
"How many games have you seen the All Blacks win in the last 20 minutes? Plenty. And that's because they practise it. We practise it now, too. We've got the base now to be able to do that.
"We've been looking at a bit of data and, conservatively, we've improved 40 per cent. Conservatively. And we've still got another 20 per cent to go."
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