"We have to back up our performances before we can talk about any World Cup... we just have to get better," he told British media.
"We have to keep our eyes on the road, because when you take your eyes off the road other things can happen. It's a challenge at the weekend and we have to prove to ourselves that we can be consistent."
England made 181 tackles and won despite having less possession than Ireland, and Mitchell said their defensive solidity was a platform for the team going forward.
"It's infectious when you get it right," Mitchell added.
"Defence reveals how much the team means to you and how much those around you mean to you - how much you care about them. It's not just the tackle, it's what the pressure creates."
(Reporting by Shrivathsa Sridhar in Bengaluru; Editing by Peter Rutherford)
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