CASTLE ROCK — After four decades of leadership, including 23 years in city government, Mayor Barbara Larsen said she’s no longer healthy enough to lead the city.

Larsen, 73, submitted her resignation letter for Monday’s City Council meeting though she did not attend due to recent heart-valve surgery. The resignation is effective at the end of the month, and the council will appoint her replacement in January.

“Every time I think about city things, my heart just starts beating faster,” Larsen said Friday. “I just don’t think I can do it anymore.”

Larsen has spent the past 44 years on the school board, City Council and county organizations, such as the county’s health advisory board. She’s also served on the Lower Columbia Community Action Program board and was a longtime foster mother who specialized in taking sick and drug-affected babies.

Larsen served on the City Council from 1984 to 1993 and decided not to seek re-election due to temporary health problems. In 1994, she was selected by the council to fill a seat vacated by John Kleine.

She was elected mayor in November of 1995 and was most recently re-elected in 2007, defeating challenger Charles Jones with 61 percent of the vote.

Larsen first discussed retiring in September but hesitated, saying she still wanted to serve the city.

Even last week — after undergoing heart-valve replacement surgery earlier in the month — Larsen said it pained her to think about stepping down.

“I kind of think I will, but I hate to do it. It’s hard,” she said.

Doctors’ advice, combined with her own soul searching, didn’t leave Larsen many other options though, she said. The decision was one of the hardest things she’s done as mayor, Larsen wrote in her resignation letter.

Former councilman Ed Smith said he’s known Larsen since he was a child and she’s always been “the sweetest lady” and a dedicated volunteer.

“She’s the epitome of what it takes to run a small community,” Smith said.

Mayor Pro Temp Greg Marcil, who has served eight years, said he can’t imagine the council without her.

While she can’t handle regular council meetings, Larsen said she plans to remain involved in town affairs.

“You’ll still see me around,” she said.

Larsen has missed more meetings in recent years due to health problems, with Marcil filling in as mayor in her absence. Marcil will serve in the post until the end of December, when he leaves the council with Khembar Yund. Neither sought re-election.

January will bring several changes to the council. In addition to a new mayor, Glenn Pingree and Ray Teter join the council in 2010 after their November election wins. The remaining councilmen, Mike Mask, Earl Queen and Jack Reilly, joined the council in 2007.

In Castle Rock, citizens elect their mayor directly, unlike other cities in which councilmembers pick a leader from among their ranks. Larsen’s term ends in December 2011, but the appointee will only serve through next November’s general election and would have to run for election if he or she wanted to serve out the remainder of the term.

Letters of application for the mayor post are due to the city by Dec. 28. Applicants, like any candidate, must be registered voters who live within the city limits.