Speaking ahead of his 300th match this Sunday against Adelaide at AAMI Stadium, the 33-year-old felt injuries and the speed of the elite level were starting to get the better of him.
Simpson had major shoulder surgery in September and stood down from the club captaincy in November, but after completing close to a full pre-season has been one of North's best players in 11 games this year.
"I thought I might get there [to 300 games] but I thought I would stumble over the line," he said.
"The game's changed ridiculously since I started but in the last two to three years it's gone to a different level.
"There isn't one minute of the game where you shouldn't be somewhere as a player these days. I've probably had to rely on my footy smarts a bit to get by.
"I don't know how long that's going to last. It might last another two weeks, it might last [until] next year. I'll take it as it comes."
Simpson said he doubted his immediate future as recently as five weeks ago and considered the press conference for this milestone could be his last one.
"I'm sure I've got people with honesty around me that will tell me how it really is at the end of the year and I'll assess it then," he said. "If there's the slightest inkling that I'm dropping away, I'll wind it up.
"People who say they don't think about milestones are kidding themselves because you do. Now that it's here I'm rapt and hopefully we have a win."
Drafted by North in 1993, Simpson waited until '95 to break into the talent-laden line-up and then played every game of the club's '96 premiership campaign.
A valuable member of another flag-winning side in '99, his personal achievements lifted again with a best-and-fairest award and All-Australian selection in 2002.
Simpson then succeeded Anthony Stevens as captain from 2004.
"With the nineties and the success we had with that group, I was sort of there for the ride a little bit early on," he said.
"The second part of my career I was the captain and at the front line, trying to help the team succeed on and off the field and keep the boys together."
Reflecting on his tenure as skipper, Simpson said the 2006 season – which saw North win two of its first nine games, finish 14th and lose their clubrooms in a fire – galvanised the group ahead of a preliminary final berth in 2007.
He added that rebounding from those disappointments reflected the culture of the club, which over the years had been crucial to his personal development.
"I know there have been some incidents over the years, recently as well, that some people on the outside question the culture and morals of our club," he said. "But I've learnt a lot here.
"Whether you've been here for a year or 15 years, when you actually leave this place you're a better person for it."
Simpson joins Glenn Archer (311 games) and Wayne Schimmelbusch (306) as the only Roos to have reached 300 games but said what those players had contributed put them on another level.
