San Francisco is a great walker’s town, and Sunday is a great day for walking. It is also the opening of the new San Francisco Crosstown Trail, which runs 17 miles from one end of the city to another.

The Crosstown Trail runs from the Candlestick Point State Recreation Area near the old ballpark site diagonally across the city to Lands End on the edge of the Pacific Ocean.

It’s part woodsy trail, part city streets, part park. It runs over hills and dales, through a mysterious forest, a swale or two, past Stow Lake and little-known Lobos Creek. It goes through Little Hollywood, Golden Gate Park and the Presidio, the Philosopher’s Way trail in McLaren Park, through parts of the city that even the most intrepid urban explorer has never seen. It’s open to everyone, even bike riders. (To see a map, click here).

The Crosstown Trail got its start with the adoption of the recreation and open space element of the city’s general plan just over five years ago. It was clear from looking at a map that the city’s parks and open spaces were all close together — so that the elements of a crosstown trail were hidden in plain sight. It was just a matter of connecting the dots. Of course, it is an article of faith that San Francisco’s government is slow, bureaucratic and impossible to deal with.

But there are real civil servants out there and they will act on a good plan — provided they get the proper push. “What we have are activist people,” said Bob Siegel, chairman of the Crosstown Trail Coalition, which includes at least a dozen neighborhood and civic groups.

This is opening weekend, and it’s a bit of a soft opening. Not all of the trail is clearly marked. Trail users will need a smartphone to download maps from www.crosstowntrail.org. Tough walkers can do it in a day. Or in two days, or in segments at their own pace.

Siegel led me on some of the segments. We started at Sunrise Campsite at Candlestick Point State Recreation Area right on the edge of the bay. It’s an obscure state park, little used but full of potential. It has an edgy feel, but the most dangerous creatures when we were there were armies of squirrels.

To save time, we drove on the city street part of the trail to Leland Avenue, the main shopping street of the Visitacion Valley neighborhood. Here, six small parks lead up the hill six blocks to Tioga Avenue. This is the Visitacion Valley Greenway, an oasis of green, of park benches, of gardens and playgrounds on what was a right-of-way for an underground city-owned water supply system.

Fran Martin and Anne Seeman were our guides here. They talked about how a series of overgrown vacant lots full of weeds, trash and stray dogs was transformed into an urban garden through the heart of a neighborhood. The greenway has been a 20-year project, part volunteerism and part through grants from the Trust for Public Land and other groups and the city Public Utilities Commission. Now it’s a green showplace produced by hard work and volunteer effort. “Look at this,” Martin said at a plant by the side of a trail. “The largest geranium in the world. And that came from a tiny cutting.”

We skipped the Crosstown segment leading through McLaren Park. Time for that on another day. Instead we drove to the Laguna Honda Hospital and Rehabilitation Center. Here we met Nick Bear, a member of San Francisco Urban Riders, a bicycle group that favors off-road cycling opportunities in the city.

He was on foot with his 14-month-old son, Tomer, in a pack on his back. Bear led us around a trail looping around the Laguna Honda grounds, through a forest of brush and trees. The trail had been there for years, overgrown and forgotten. “This was just a blank spot on the city maps,” Bear said.

Now, it’s a path through a small urban forest in the middle of the city. At one point Bear pointed out the headwaters of a tiny creek. At another the remnants of a dump site once used by the hospital. Ancient trash. “We call this “Bedpan Alley,” Bear said.

We went up the hill, through groves of trees to the top at Panoramic Boulevard. The view from the top of the hill is breathtaking. So is the trail. It’s all uphill.

Down again, past the Ruth Asawa School of the Arts, through Glen Park, and on two short streets and then to other city-owned space, now transformed into the Glen Park Greenway near the BART station.

We had crossed several other trails leading to various parts of the city, and even the ridge trail, which connects to more than 350 miles of Bay Area trails.

“For me,” Siegel said, “this is the golden age of trails.”

Some groups are leading fundraising guided walks on the new San Francisco Crosstown Trail in the next few weeks. But the trail is free and open to anyone willing to walk or bike it.

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