"This is an amazing resource you have here," a newcomer from Indianapolis said as he paused during a recent mountain bike ride in Point Mugu State Park on the western edge of the Santa Monica Mountains National Recreation Area. "It's wonderful." And a bit of a wonder. The world's largest urban national park, which officially turned 25 on Monday, stretches 46 miles from the Hollywood Bowl to Point Mugu in Ventura County and snakes along the coast from the Santa Monica Pier past Malibu. (The National Park Service boasts that the park encompasses more Zip codes -- 26 -- than any other in the system.) But that doesn't mean all 153,075 acres are on the wild side. Some 75,000 acres are recreational parkland, such as Point Mugu and Malibu Creek state park, but the rest includes odd bits and pieces (the Park Service shares jurisdiction with more than 70 government entities), including a military site, Malibu, private homes, city and regional parks -- which makes you wonder how it ever came together. To mark the anniversary, the Sierra Club has organized Sunday hikes in November that feature the Backbone Trail. "We just want people to come out and take a look," said Milt McAuley, 84, who has led such hikes for more than 20 years. On Sunday, you can tough out a strenuous 18-mile hike with 3,500 feet of gain along a loop from Tapia Park to Castro Crest and back, or take an 8-mile hike from Stunt Road to Tapia Park. The park service is repeatedly showing a 22-minute film about the park, "Mountains, Movies, Magic," at an open house from 9 a.m. to 5 p.m. Saturday at 401 W. Hillcrest Drive, Thousand Oaks. For open house information, call (805) 370-2301. For hike start times and trail heads, call (213) 387-4287.