The 68-year-old stepped down Monday after Boston eliminated the Tigers from the best-of-seven American League Championship Series with a 5-2 win in Game 6 on Saturday night.
"I'm going to be 69 years old," Leyland said at a news conference in Detroit. "I'm not ashamed of that. I'm proud of it. The fuel's getting a little low."
Detroit won the AL Central division three years in a row and two AL pennants under Leyland, who spent eight seasons with the Tigers.
He had been working under one-year contracts the past couple years and was content to wait until after the season to address his status.
He was a bit reflective late this season, mentioning to reporters that he had already managed the Tigers longer than he had expected they would keep him, but he also said in September that he still loved the atmosphere, the competition and his team.
Leyland has been a manager for 22 years, compiling a 1,769-1,728 record with the Tigers, Pittsburgh Pirates, Florida Marlins and Colorado Rockies. Leyland won the 1997 World Series with the Marlins.
Leyland was 700-597 with the Tigers. He led them to the World Series immediately after taking over in 2006, losing to St. Louis in five games. The Tigers went to the World Series again in 2012 but were swept by San Francisco.
This year's team had a chance to make it back, but Detroit couldn't take advantage of its standout starting pitching against the Red Sox.
"This one hurt bad, because I thought we let one get away. We did it collectively, there's no one culprit," Leyland said. "This is one that's going to stick with me."
When Leyland arrived at the Tigers' training camp this year, it marked 50 seasons since he first showed up there as an 18-year-old prospect. His playing career never amounted to much, but his accomplishments as a manager over more than two decades have been impressive.
Leyland has also been part of some of baseball's most memorable games — Pittsburgh's loss in Game 7 of the 1992 NLCS at Atlanta, Florida's victory over Cleveland in Game 7 of the 1997 World Series and Detroit's loss to Minnesota in a one-game playoff for the AL Central title in 2009.
When he took over the Tigers, they had gone 12 years without a winning season. Under Leyland, they finished under .500 only once.
"What's gone on here has been unbelievable. We've won a lot of games, we've had a lot of seats filled," Leyland said. "I came here to make talent a team, and I think we did that."
Detroit has become one of baseball's glamour teams of late, with stars like Miguel Cabrera, Justin Verlander and Max Scherzer helping the team win games and draw fans. The Tigers should be able to keep their core of players mostly together for next season, but now they'll need to find a new manager to replace Leyland, who always earned high marks for his ability to keep his veteran team focused.
"I truly think this is going to be a very good team next year," Leyland said. "This job entails a lot more than people think."