Tuesday was one of those days that sometimes shows up in midwinter in San Francisco. It was warm, the sky was a deep blue and swept clean by a weekend windstorm. It was a perfect day for a walk in the park. So I took a walk in John McLaren Park with some of the park’s friends.

McLaren Park, out in the southeastern part of the city, is a mystery to most San Franciscans. It is the third-biggest park in town, 312 acres. Only Golden Gate Park and the Presidio are larger. McLaren Park has five playgrounds, two lakes, a mysterious marsh, hundreds of trees, a long and winding trail devoted to philosophers, a nine-hole golf course — and a bad reputation.

It was the place where the bad guys dumped bodies, where criminals lurked behind the trees. Linda Stark Litehiser remembers when she’d invite friends for a picnic in the park. “‘Oh no,’ they’d say. ‘Not there. It’s dangerous.’ Even the cops warned us about it.’’

But you should see McLaren Park now. It still has that open, remote feel. “Like being in the country,” Litehiser said. But as a result of work by the city and by park advocates, the park has been transformed. It’s never really crowded, but there are plenty of people on the trails, in the playgrounds and at the city-owned Gleneagles golf course on Sunnydale Avenue.

“It’s one of those true hidden treasures,” said Andre Cato, who lives in the Visitacion Valley neighborhood and walks in the park almost every day.

Litsehiser led a tour of McLaren Park last week, up hill and down dale, talking about its virtues all the while. She is a member of the McLaren Park Collaborative, an umbrella of park advocacy groups.

She sees it as a neighborhood park for the southern corner of the city, the Excelsior, Crocker Amazon, Portola and Visitacion Valley neighborhoods. They are some of the last affordable parts of San Francisco. But it’s also a regional park, close to freeways and offering a good urban park experience.

Advocates particularly like the Jerry Garcia Amphitheater in the center of the park. It’s a full-service event venue with grandstands, dressing rooms and all the trappings.

The amphitheater, which opened in the 1970s, was underused for years, but Tom Murphy, who grew up a block from the park, put it on the map in 2002 when he started the yearly Jerry Day to celebrate the life of the Grateful Dead’s Jerry Garcia, who was born and raised in the Excelsior.

The amphitheater can accommodate more than 2,000 people in the seats and on the grassy hillside. Besides Jerry Day every August, there are other performances, including the San Francisco Mime Troupe and a big-band concert.

Litehiser led the way up winding trails to the small Upper Reservoir, which is lined with trees and reeds, as serene as any mountain lake. It is a favorite for dog walkers and strollers. A brown mixed-breed dog was swimming in the lake despite a “No swimming” warning. “He didn’t see the sign,” the dog’s owner said. “He can’t read.”

Joe Litehiser, Linda’s husband, swears that big fish sometimes inhabit the small lake. There were no fish visible the other day, but a turtle was spotted on a lakeside rock, basking in the winter sun.

Next stop was the top of the hill under a landmark blue water tower on the western edge of the park. There is a spectacular view there, east to Mount Diablo in Contra Costa County, north to Mount Tamalpais in Marin County, and west to the Farallon Islands, more than 30 miles away. The towering skyline of San Francisco glittered to the northeast, close but a world away.

The blue tank is near a way point for the 2.7-mile Philosopher’s Way, a trail that leads around the edges of the park, with granite stations where strollers can contemplate ideas and, perhaps, the view.

“We want to make it nice up here,” said Rose Ann Harris, who lives in the Excelsior and volunteers in the park. “We want to make it a community for everybody.”

Murphy, the man behind Jerry Day, credits park advocates like Chuck Farrugia, who lobbied hard with the city to get more support and city staff for McLaren Park.

And he credits head gardener Daniel Choi. “By far the MVP of McLaren Park,” Murphy calls him.

The Litehisers brought the group past murky Yosemite Marsh, down to McNab Lake near the northern edge of the park to show off the lake, the birds that thrive there and a mosaic built by more than 75 artists and neighbors a couple of years ago. It features a butterfly and three long garter snakes in red, blue and black tile.

Then it was off past a community garden to Gleneagles golf course, a nine-hole course that regulars say is very tough. But it’s not so tough to have a beer at a little out-of-the-way pub in the clubhouse.

It’s one of those hidden urban gems.

“People in the know know about this place,” Linda Litehiser said. “But they keep it to themselves.”

Carl Nolte’s column appears Sundays. Email: cnolte@sfchronicle.com Twitter: @carlnoltesf