Kellaway was named Rugby Australia's 2021 rookie of the year on Friday after a dream debut Wallabies campaign in which he scored nine tries in 13 Tests.
The 26-year-old was considered a bolter in Dave Rennie's 38-man squad for the three-Test series in France last year, after only having just returned to the Melbourne Rebels from a stint in Japan.
But it didn't take long for him to establish himself as a regular in the Test side and build a reputation as a try-scoring machine.
Kellaway felt coming into the Wallabies' set-up as a 25-year-old meant he had the tools he needed to succeed.
"If I was thrust in at 21, in hindsight I probably was thrust into the Waratahs team that was absolutely stacked with Wallabies (at that age), so one could argue it was not too far off," Kellaway said.
"I look back and I look at the performances and how involved I was in those (Waratahs) games, and I was a bit of a passenger.
"I was happy to be there rather than happy to compete.
"I look now, and the clock is ticking. There's not much longer left for me.
"I might be getting rookie of the year, but I'm still 26 going on 27.
"In terms of your rugby timeline, that's definitely ticking for me."
Kellaway said being a Wallabies bolter last year meant there was not much pressure on him to perform.
"I was quite lucky in a sense that I didn't have anything to lose," he said.
"As Dave (Rennie) said, I wasn't in the plans or any of that.
"Being called in was good enough at that point. Expectation was slim to none.
"They're always good circumstances to try to perform when nobody expects anything of you, and you've got a little bit of a chip on your shoulder and trying to prove people wrong."
Kellaway's full attention is now back on the Rebels and helping them crack through for a finals berth in the inaugural Super Rugby Pacific season.
"If we're honest, (we're) probably serial underachievers," Kellaway said.
"We've had, at least from where I see it, a pretty decent roster for the last four years and haven't quite been able to crack the consistency we need to play finals footy."
The Rebels' campaign begins against the Queensland Reds at Suncorp Stadium on February 19.
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