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."
|Shakhtar Donetsk 2 Hoffenheim 2: Maycon denies Nagelsmann´s side|
|Ajax 3 AEK Athens 0: Tagliafico at the double in opening win|
|Dalot makes debut, Martial starts for Manchester United|
|India avenge ICC Champions Trophy loss with big win over Pakistan|
|Ronaldo makes Champions League debut for Juve|
|Lopetegui picks Navas over Courtois for Champions League clash with Roma|
|Willian very happy under Sarri after Conte row|
|Sarri deals blow to Chelsea kids´ Europa League hopes|
|Mendy out for Manchester City as Aguero makes bench|
|Cech accepts performances key to securing Arsenal stay|
|Monchi rules out Spain return amid Barcelona rumours|
|Durham skittled for 61 and 66 on the same day|
|Kane isn´t a machine – Lamela defends misfiring Spurs star|
|Hazard left out of Chelsea squad for PAOK trip|
|Tiger feels close to ending drought after achieving Tour Championship goal|
|Leno in line for Arsenal chance in Europa League|
|Gazidis leaves Arsenal in good shape, insists Emery|
|Eagles bring back WR Matthews, place Wallace on IR|
|England include Stokes, Hales and uncapped seamer Stone in ODI squad|
|Suwon Bluewings 0 Jeonbuk Motors 3 (aet, 4-2 on penalties): Quarter-final comeback ends in spot-kick|
|Australia captain Paine ready to slog it out with Pakistan|
|Nadal pulls out of China Open & Shanghai Masters with knee injury|
|Ostapenko begins Korea Open defence with victory|
|Joshua v Povetkin: How Olympic super heavyweight champions have fared|
|Liverpool ´made a point´ with PSG victory - Gomez|
|Osaka wows Tokyo crowd on WTA return|
|Excited Povetkin arrives in England for Joshua showdown|
|Kovac patience with Sanches set to reap rewards|
|Kovac: Madrid strong without Ronaldo, Messi is the best|
|ATP suspends umpire Lahyani over Kyrgios ´pep talk´ at US Open|
|North and South Korea seek to co-host 2032 Olympics|
|Ampadu signs long-term Chelsea contract|
|Don´t blame Neymar, PSG players all guilty - Thiago Silva|
|Real Madrid seek to plug Ronaldo scoring gap – Champions League in Opta numbers|
|Brewers allow Cardinals to inch closer with loss to Reds|
|Manchester is still red – Depay ahead of Lyon´s clash with City|
|Orioles become first pro team to wear braille jerseys|
|Klopp hails ´brilliant´ Henderson|
|PSG attack lacked aggression, says Tuchel|
|Atletico Tucuman 0 Gremio 2: Libertadores holders draw first blood in QFs|
|Thomas plays down wrist worry|
|Rakitic wants more from Barcelona hat-trick hero Messi|
|My players aren´t s*** – Pochettino defends Spurs squad|
|Cubs reliever Morrow won´t return this season|
|Tebow expected to be back in Mets organisation next season|
|Inter can compete against anyone – Icardi revels in Champions League victory|
|Tuchel: PSG loss to Liverpool not logical or correct|
|Sturridge in best shape I´ve seen – Klopp|
|Milojevic celebrates ´fantastic´ occasion after Red Star´s 26-year wait|
|Rodrigo and Simeone salute Griezmann display|
|Seahawks DE Clark on Bears QB Trubisky: He´s nothing special|
|Carroll says Wilson has been ´over-trying a little bit´|
|Judge returns to Yankees´ starting lineup|
|Napoli boss Ancelotti takes aim at Red Star time wasting|
|Favre admits young Dortmund stars struggled in Bruges|
|Otavio would trade goal for Porto victory as he bemoans Schalke draw|
|Championship Review: Pacesetters Leeds crush Preston, Villa end five-game winless run|
|Neymar world-class but not a defender - Klopp says Liverpool targeted PSG star|
|Waiters injury update: Heat guard unlikely to be ready for start of season|
|Gojowczyk eliminates returning Tsonga in Metz|
|Liverpool enjoying best start in 57 years after PSG win|
|Galatasaray 3 Lokomotiv Moscow 0: Terim´s side make light work of Russians|
|Club Brugge 0 Borussia Dortmund 1: Super sub Pulisic scores late winner|
|Schalke 1 Porto 1: Fahrmann saves one penalty, but Otavio scores another|
|Red Star Belgrade 0 Napoli 0: Points shared as Italians fail to make possession pay|
|Liverpool 3 Paris Saint-Germain 2: Firmino strikes late as Reds edge five-goal thriller|
|Monaco 1 Atletico Madrid 2: Costa and Gimenez seal comfortable win|
|Chaos reigns in Serie B as league denies suspension claim|
|India secure Asia Cup victory despite valiant Hong Kong effort|
|Messi makes "extraordinary routine", says Valverde after Barca star surpasses Ronaldo|