Wales headed to Poland for two punishing trips to hone the squad's fitness now underpinning the bid to reach the later stages in the tournament.
Victory over Namibia and Fiji would now secure a potential quarter-final appointment with Ireland in Wellington on October 8.
And Charteris admits the gruelling summer schedule was worth all the pain.
"It's in our hands now, we've got to go out and beat Namibia and we need to win with a bonus point," Charteris said.
"Looking back to the summer, we really didn't want to be in Poland. It was horrible out there, and we were cursing the coaches.
"But we felt the difference in the last 20 minutes against England, and we finished that game (in Cardiff in August) on a high.
"We have still got something left in the tank near the end, and that is when games are won and lost."
Wales will return to training on Wednesday, with Chareteris admitting the squad need to take their chances if they are to build on their promising start to the competition.
"We need to finish the chances off in the next two games because if you give these teams a sniff they can break you. We've got to be more clinical and ruthless," Charteris added.
"There were some pretty battered and sore bodies around after Sunday. It was a brutal one, but we got the win.
"We know our game-plan didn't work in the first half and we didn't execute it as well as we wanted, but we changed tactics at half-time and we got the reward.
"If you want to win it (the World Cup) you've got to beat the best teams, and we showed against South Africa we can mix it with the so-called superpowers.
"We have the character and the attitude that if we go behind we never panic. We have always had the confidence in our conditioning, and we wore Samoa down."
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