After Shaun Wright-Phillips' opener, Wayne Rooney scored his first competitive goal for England in more than three years, then an own goal from Taavi Rahn completing the scoring after 34 minutes.
"We didn't perform well in the second half, I can understand it will be flat, we're going to be thinking about Wednesday.
"I can't criticise it - I watched Croatia play Estonia and go 2-0 up and stroll the second half to conserve the energy. That's what we did."
Man-of-the-match Gareth Barry maintained the hard work still lies ahead.
He said: "It is a great feeling to be named man of the match here at Wembley on the back of a great team performance and victory.
"Things are looking good at the moment but we know it is only half a job done because there is another big game - probably the biggest for a long while - coming up in Russia.
"The Russians have to win the game and so we have to go there and be positive to get the three points.
Rooney added: "We have got the job done today and we've got a similar game in Russia to the one we had to tackle in Istanbul a few years ago.
"We know it will be difficult and it will be a tough game - but we've got to go out there and attempt to do the job we know we have to do.
"It was a nervy night over there in Turkey. We had a few chances and could have won it. But we went out there and did a job and I am confident we can do the same again.
"There is definitely the belief and quality in this squad to get the same sort of result. There are a few younger players in the squad now - but the same, if not more, quality."