"I came to that realisation when I made the decision and I knew I could steel myself for another couple of challenges," he said. "And now I walk away comfortable with my decision and looking forward to the next stage of my life and spending some time with my family and taking up some other challenges as well."
McCullum thinks the New Zealand team are ready for a new leader, "and a slightly different direction". His hope was that he had brought fun and enjoyment back into the setup but acknowledged there were times when the players felt "vulnerable" under his direction. It came with risks - often with spectacular results, but equally some let-downs - and this was illustrated in his final test.
His fastest test century on Saturday was a wonderful way to bow out, in the individual sense. Losing the match, and series, wasn't the ideal departure but "the right team won".
McCullum's final match produced a series of standing ovations, a guard of honour from the Australian opponents and just finishing up at home - even if Christchurch is his adopted home - mattered.
"You never foresee that level of respect that Steve Smith and the Australian team showed with that guard of honour would ever eventuate," he said. "Having said that, it was a nice touch and very nice touch from our boys today as we went out on the park, in the changing room, away from the cameras.
"We had a very similar guard of honour, I guess, which was very nice. Memories which I'll take with me forever."
As McCullum addressed media for the final time, perhaps the most revealing thing was when he pinpointed the time New Zealand changed as a team, reset their parameters and how they wanted to play the game, how they wanted to make a difference. It also made them the flagbearers for playing cricket with both a smile and soul.
It came in Sharjah in the United Arab Emirates, late in 2014. It was a test against Pakistan which no one wanted to play, in the immediate aftermath of the death of Australian player Phil Hughes, who was struck on the head in a Sheffield Shield match.
It was a match played with complete abandon. Cricket was secondary. New Zealand won by a mile but, within those five days, was born a new attitude.
"Sharjah was a huge moment for us as a team. That was a catalyst for a lot of the free-flowing cricket we've seen, without the inhibitions of worrying about what may go wrong. We almost stumbled, through the tragedy, on a style of play for us which kind of worked.
"When you've got to ask a team to play a game during those circumstances, I think it galvanised us as a group. I look back proudly on how our guys stuck together and built the soul of our group during that tough time."
McCullum is sure he's leaving the team in good hands. Enjoyment and culture is "spot on" but he's picking more professionalism will emerge.
"I'm probably a bit more old school. The next stage for this team is to reach a high performing level on a consistent basis."
The last contributions McCullum made in the test today were a final catch and a last referral for an lbw, symbolic of how much the game has changed throughout his career.
So how would he like to be remembered? "It doesn't bother me, to be honest."
Then he thought again.
"You hope you're remembered as a guy who played with a lot of passion, a lot of pride in playing for his country, and played the game for the right reasons as well - and who, if in doubt, was prepared to take the positive option."
He's looking forward to "a bit of golf, a bit of racing, probably play some darts, just spend some time with the family and take a breath as well". His black cap will take pride of place "somewhere".
"I'm not a sentimental sort of bloke."
Before he left, he offered a final offering: "Hopefully the guys that I've played with will remember you as a good bloke as well."
