"The stamina just isn't there. The arm strength came back ... but it wouldn't last," Zaun said Monday morning. "After three innings yesterday, I was totally gassed. The amount of time for me to get ready to play at a high level ... it just saps the fun out of it."
Following Sunday's game against the A's in Peoria, Ariz., Zaun walked into the office of Padres manager Bud Black and informed him of his decision to retire.
"The first thing to me was that I wanted to come back and be completely healthy and not just to catch once or twice a week," Zaun said. "But I didn't think it was going to happen in three weeks.
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"I owe the Padres more than that. They gave me a great opportunity and I didn't want to leave them hanging. If I was there, I would be taking time away from the other guys in camp."
Zaun, 39, was in camp on a Minor League contract. The Padres were hopeful he would win the backup catching job behind Nick Hundley.
But the right shoulder Zaun had major surgery on last June -- which ended his season with the Brewers after he tore the front and back of his labrum -- recovered well enough for him to make a run at another season.
Once in camp this spring, though, the shoulder was slow to get loose and would eventually lose strength with the more work Zaun did.
"I didn't think I would get to the point where I would be dependable for them," Zaun said.
Zaun, a switch-hitter who batted .252 in 16 Major League seasons, said another contributing factor in his decision to retire was he didn't have the same zest for the game that he once did.
"I don't have the same enthusiasm. I enjoy the guys and being in the clubhouse each day, but when it came down to the playing end of it, I could take it or leave it," Zaun said.
Still, Zaun said, he'll walk away from the game with no regrets. He earned a World Series ring with the Marlins in 1997 and made countless friends along the way.
"I've got a World Series ring," Zaun said. "I've had a great run. I feel really good about my decision. I'm really excited about the next step."
The next step, Zaun said, is to get back into broadcasting. He has served as a postseason analyst for Rogers Sportsnet in Canada since 2006.
"Broadcasting is the next step," he said.
Rob Johnson, whom the Padres traded for in December, now appears to be the frontrunner for the backup catching job. The Padres have Kyle Philips and Guillermo Quiroz in camp. Both have Major League service time and both have performed well so far in camp.
San Diego general manager Jed Hoyer said the team will look at internal options but will also keep an eye on options outside the organization.
Corey Brock is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.