San Diego State reached its first ever Sweet 16 in March, which could have been a perfect exit for longtime coach Steve Fisher.

Instead, the 66-year-old Fisher has agreed to a four-year extension with the Aztecs, a source with direct knowledge of the situation told ESPN.com Sunday night.

San Diego State will make the formal announcement at a 5 p.m. ET news conference Monday, according to a source. Fisher couldn't comment Sunday night but did tell ESPN.com that his health is great. Fisher had prostate cancer and subsequent surgery for it May 17, 2008, but said his blood tests for the disease were all good at his last checkup.

According to the San Diego Union-Tribune, there's language in the contract that makes Fisher's longtime assistant Brian Dutcher the "coach in waiting."

Fisher is the longest-tenured coach in the Mountain West, joining the league in its first season in 1999-2000. Fisher is the only remaining coach in the league from that season. He began a remarkable turnaround of a once-dormant program, taking the Aztecs from an 0-14 MWC season in 2000 to an NCAA first-round appearance in 2002 by winning the conference tournament. The Aztecs then won the league in 2006 and reached the first round. San Diego State won the conference tournament again in 2010 but lost in the first round of the tournament, setting up last season's magical run.

Fisher's team went 34-3 overall, losing twice to BYU and once to eventual national champion Connecticut in the Sweet 16. San Diego State was 14-2 in the MWC.

Fisher, who famously coached the 1989 Wolverines team to the national title after the late Michigan athletic director Bo Schembechler fired Bill Frieder for going to Arizona State, eventually coached Michigan's Fab Five to consecutive national title games in 1992 and '93. Fisher was fired in 1997 in the aftermath of a scandal involving Ed Martin, a retired Ford electrician who was giving athletes illicit benefits. Fisher didn't receive any sanctions by the NCAA and two years later was hired by San Diego State.

The Aztecs had never won a tournament game before last March.

San Diego State lost quite a bit of the Sweet 16 team with four starters gone -- seniors Malcolm Thomas, D.J. Gay and Billy White and junior Kawhi Leonard, who left for the NBA draft and was selected by the Indiana Pacers at No. 15 before being traded to the San Antonio Spurs. The Aztecs also learned that senior Brian Carlwell didn't get a waiver to play a sixth season after being sidelined for a year following a car accident when he was at Illinois.

The Aztecs do return two key players who rotated as fifth starters in James Rahon and Chase Tapley. They also added LSU transfer Garrett Green for this season since Green graduated and can earn a waiver to play immediately as a graduate student.

The Aztecs likely will be picked third or fourth with Colorado State in the MWC, behind co-favorites UNLV and New Mexico.

Last week the school announced that Fisher and BYU coach Dave Rose will coach a pair of exhibition games featuring NBA draftees on Sept. 21 and 22 in Salt Lake City and Provo, Utah, with Leonard and BYU's Jimmer Fredette two of the players committed to the event.

Fisher is the same age as Gary Williams, who retired from Maryland in the spring, and three years younger than UConn's Jim Calhoun.

Andy Katz is a senior writer for ESPN.com.