07/09/2012 at 03:00 PM EDT
Michael Phelps is well aware that he's a veritable "old man" in his sport as he approaches his final Olympic Games
"At this point in my career, everybody has caught up," the swimmer (who won 16 Olympic medals between the 2004 and 2008 Games) tells Details
magazine in its August issue. "So I'm fine-tuning the little things that add up to make a huge difference."
One thing he's not adding up? "I don't count calories," Phelps, 27, says of his not-too-strict diet. "Whether it's Sour Patch Kids or Reese's or a bag of chips, if I feel like eating it, I'm going to eat it."
In addition to his athletic success, Phelps also gained notoriety for the infamous marijuana pipe photo
from 2009. "It was a learning experience," he tells the magazine.
"I'm the kind of person who has to go through the learning experiences myself. Somebody could tell me, 'If you eat this much you’ll be fat,' and I'd be like, 'Yeah, okay, let me try it.' I'll be the first to say I've made thousands of mistakes, but I've never made the same mistake twice."
As for the upcoming Olympics, "I realized that I probably hadn't reached my full potential," says Phelps. "There was still more in the tank. As I come to closure on my career, am I going to look back in 20 years and say, 'What if?' That's something I don't want."
The August issue of Details
hits newsstands Wednesday.