The Rachel Carson Trail is a very long bit of a miracle, a 34-mile stretch through the top of a county of 1.3 million people that can still leave a walker entirely alone.

After walking the western fifth on a recent afternoon, from Indiana Township to North Park, hopscotching across the stones in Crouse Run and ascending and descending blessedly shaded slopes, I was surprised to read in the hiker's guide: "This is the easiest of the four sections of the trail.''

I'd have spat were I not so dehydrated.

Next Saturday, some 600 people will attempt to walk or run the whole trail within the span of daylight -- 15 hours, 4 minutes. They'll start at 5:30 a.m. in Harrison Hills Park in Allegheny County's northeast corner and walk west to North Park. It's the 12th annual challenge, a fund-raiser for the Rachel Carson Trails Conservancy. The cost is $50 and, proving that people will do anything for a cookout and a shirt, it's sold out.

The Homestead Challenge or "Half-Challenge'' starts near Springdale High School, is "only'' 18 miles, and registration spots remain available at $65. The Family Challenge is an eight-mile walk that begins at Hartwood Acres and ends at North Park, and families of up to five members can sign up for just $70. Information can be found at rachelcarsontrails.org or by calling Steve Mentzer at 412-512-4544.

I did the eight-mile stretch with Tom Armstrong of Squirrel Hill and his older brother, John, of Indiana Township. Tom did the 18-mile Homestead Challenge last year while John, 61, did the entire walk. They'll both do the full walk Saturday. Tom says he hopes the organizers improve on the trail mix of M&Ms, raisins and peanuts offered at checkpoints last year.

"Throw in some Advil for the old guys,'' Mr. Armstrong, who's 60, said.

I walk five or six miles a day with my North Side commute, our dog and just plain life, but that's spread over a long day. A 7.5-mile walk on this trail was an agreeable challenge.

We walked the mile from John's house to enter the trail at the intersection of Church Lane Road and Wagner Road. The trail is generally on private property, skirting the backs of homes. We weren't far along when we came upon Hidden Pond, where Tom said, "That'll look pretty inviting at 4 o'clock in the afternoon after walking 25 miles.''

John once saw a great blue heron arise from that pond. A little farther down the road, we passed behind another pond and he told me of the time a red-tailed hawk pulled a squawking mallard drake from the water.

On McCully Road, about a half-mile west of Middle Road, we could see the top of the U.S. Steel Tower 10 miles south, and the big roads we crossed also were reminders we were in a metropolitan area, but those moments were few and far between. Mostly we were alone as we followed a trail marked by yellow, rectangular splotches on tree trunks.

I wasn't wearing a watch, but we would easily walk 20 or 30 minutes without seeing another person. The area around Crouse Run was the most primitive, and the trail had us crossing and recrossing that creek. The alpine walking poles the Armstrongs provided made me feel like Baron Von Trapp, but served me well as we repeatedly ascended and descended.

Tom spooked a heron but he was so far ahead all I saw was a flash of color. There were deer, of course, and also, in the midst of the otherwise natural world, concrete abutments. Those could only be the remnants of a bridge for the Pittsburgh-to-Butler interurban trolley line that ran through there between 1908 and 1931. (With gasoline at $4 a gallon, I hope somebody still has the plans.)

About halfway through our walk, John left us to walk back home and get his car so he could meet us in North Park and drive us back to Tom's car in Indiana. The toughest part of our hike may have been not stopping at the Kool Cones stand in North Park so we'd get to our meeting place with John on time.

There may be better ways to spend a long day in Pittsburgh, but none would give you a better feeling for how the North Hills earned their name.