The left-handed batter, who will turn 39 during the Adelaide Test, which starts on Wednesday, missed the pink-ball encounter against England due to a back problem, and was also unable to open in either of the innings in Perth last month.
"I could have retired two years ago, or at any time," Khawaja told reporters in Adelaide on Saturday.
"I am always mindful of the future, I always have been. I am not here to hang on, I am just here to enjoy my cricket.
"As long as I am valued, I am here, I am doing my job."
In Khawaja's absence, Travis Head and Jake Weatherald have been in fine form at the top of the order.
Head scored a match-winning 123 in Perth, while Weatherald contributed with a solid 72-run knock in the second Test.
"I've got to be totally honest, it doesn't really faze me," Khawaja said on the chances of his return to the playing XI.
"The older I have got, the more comfortable I have got with things that I can control and things that I cannot.
"And I feel really good. I am ready to go.
"The rest of it is not in my control. So we will see what happens."
Australia lead the five-match series 2-0, and a win in Adelaide would secure the urn.