Courtney Lawes says an increased work ethic has been key in helping him hit top form in recent times.
The indefatigable forward has been in great shape for Saints this season, winning the Aviva Premiership player of the month for September.
He impressed for England and the British & Irish Lions last season, helping his country claim Six Nations glory and the Lions earn a series draw in New Zealand.
And now Lawes will set his sights on England's clash with Argentina at Twickenham on Saturday.
“The biggest thing for me has been being able to stay on the pitch,” Lawes said.
“My fitness is better and a lot of that is down to my work ethic in the gym and on the training field being way higher than in previous years."
England will also face Australia and Samoa this autumn.
And Lawes, who is joined in the Red Rose squad by club-mates Dylan Hartley and Piers Francis, will hope to be a starter against Argentina on Saturday.
"I still have a lot to do to get into this team; we have a lot of good second-rows, and a lot of competition for places," he said.
"(Saracens lock) Nick Isiekwe is a good player. He has some similarities to Maro (Itoje) but is a bit different in terms of game understanding.
"He is very talented, very strong and powerful so if he can stay fit and keep improving he is going to be a very good player.”
England's long-term goal is the 2019 World Cup in Japan.
And Lawes said: "We have a limited time in terms of international experience before the World Cup.
"It is on its way and we need to make everything count, but we do that by just focusing on the next game, and that is Argentina."