Eddie Jones has admitted he feared the worst after being appointed England coach as he recalled his shock at the squad's lack of fitness.
Jones took charge after tournament hosts England's woeful first-round exit at the 2015 World Cup, which included defeats by Wales and his native Australia.
He launched an England career that has so far yielded an impressive 24 wins from 25 Tests with a 15-9 win away to Scotland in the Six Nations two years ago.
But as he prepared to take his back-to-back Six Nations champions to Edinburgh again for another Calcutta Cup clash on Saturday, Jones indicated England's progress had not been as serene as it might look.
"I remember the first game, we were only together for two weeks so we only had four training runs," Jones said Thursday.
"I remember after the first training run, I have just had a look at it on video the other day, and I was thinking 'Goodness me what have I got myself into here?’"
Jones also indicated the England squad he inherited lacked tactical flexibility too.
"They weren't fit. They wanted to play a system of attack, a system of defence and I thought 'this is going to be hard work'."
- 'Couldn't run anymore' -
He added: "After 20 minutes, they were just shot. They couldn't run anymore. 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."
England are currently second in the global rankings behind world champions New Zealand and Jones said: "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."
But Jones insisted it was the players themselves who deserved credit for a run of results where the only defeat thus far took place when Ireland ended England's hopes of a second straight Grand Slam last season.
"It is hard for good players to change and it is a great credit to the players that they have been able to accept that they needed to change and they have done that," said Jones as England prepared to resume their quest for an unprecedented third successive outright Six Nations title following contrasting wins over Italy (46-15) and Wales (12-6) already this season.
The former Australia and Japan coach added: "They've probably learnt more from themselves than me. They've learnt how to play winning rugby. Test match rugby is about winning, it's not about entertainment."
Jones's leader on the field has been Dylan Hartley, with one of his first acts as Red Rose boss to appoint the hooker as England captain even though the Northampton front row has missed a staggering 60 weeks of his career through suspension and was left out of the 2015 World Cup squad.
Yet he has repaid Jones's trust, with the New Zealand-born forward set to become England's second most-capped player of all time when he makes his 92nd Test appearance on Saturday.
"He’s made me realise what I need to do and he's pushed," said the 31-year-old Hartley of Jones. "He made it very clear that if I didn't hit targets, I wouldn't be here."