ST. LOUIS — Karen Hertlein says she doesn’t know how old she is. She hasn’t kept track since middle school.

“My grandma never did, and I don’t either. She said it’s a good way to remain ageless and not worry about where you are in life,” Hertlein said.

Her husband, Jim Hertlein, has adopted this philosophy, too.

Presumably, agelessness helps when tackling feats of endurance. But for the record, she’s 47, he’s 57.

In 2006, the couple from Kirkwood, Mo., gave themselves the trail names, Mapman and Robin, strapped on 30-pound backpacks and hiked all 2,175 miles of the Appalachian Trail from the peak of Springer Mountain in Georgia to the peak of Mount Katahdin in Maine.

Two years later, the duo hit the Pacific Crest Trail, and backpacked from Mexico to Canada. It’s 2,650 miles and arguably more rugged than the Appalachian Trail.

Earlier this summer, they took a pebble from the Pacific Ocean, pedaled 3,514 miles from Anacortes, Wash., to Boston on a metallic-orange tandem bicycle and tossed that pebble into the Atlantic. All told, the Hertleins traveled 8,339 miles, powered by their own muscles.

One way to meet people

They camped with grizzled outdoorsmen, adventure-seeking executives and down-and-outers trying to find themselves on the Appalachian Trail. They trekked the Pacific Crest Trail with other hikers nicknamed “Lumbar,” “Milky” and “Forrest.” And they broke bread with strangers who saw them cycling on the side of the road or resting outside a convenience mart and invited them to sleep at their homes. Francis, a rancher in Nashua, Mont., still makes the Hertleins wistful.

They had pedaled 18 miles from his house the following morning when the old rancher pulled up beside them in his pickup, tears glistening in his eyes.

“I just didn’t get to say goodbye enough,” he said, giving them both hugs after waving them over.

They got the urge to hike the Appalachian Trail about four years ago when they saw a documentary on PBS about hikers who finish the entire trail in one season.

It takes about 5 million footsteps to cover the entire length of the Appalachian Trail, and more than that on the Pacific Crest Trail. According to the Appalachian Trail Conservancy, elevation along the trail rises and falls so frequently and steeply that hiking it is equal to climbing up and down Mount Everest 16 times. Thousands of people try to hike through each year. Only one in four makes it.

The Hertleins averaged 16 miles a day on the Appalachian Trail, 22 miles a day on the Pacific Crest Trail and 95 miles a day when cycling.

Susan Petroff, one of Jim Hertlein’s co-workers, hiked for two weeks with the Hertleins on the Appalachian Trail.

She was on fresh legs when she caught up with them near the halfway point of the trail in Harper’s Ferry, W.Va., but the Hertleins insisted on carrying some of her food until she got used to her heavy pack. “They made me feel like they weren’t going to leave me in the dust, which they easily could have,” said Petroff, 48.

‘Isn’t this fun?’

The Hertleins established three rules before they set foot on their first odyssey, the Appalachian Trail: No. 1: Read the Bible every day. No. 2: Never say “I can’t.” And No. 3: Don’t whine or complain to the other guy.

Petroff recalls a long, monsoonlike downpour when she, the Hertleins and some other hikers were all soaked to the bone and forced to hike in water shin-deep for several days. On the third day, she said, they reached a shelter that was built to accommodate six people but nine hikers squeezed under it.

“Karen turned to me and said ‘Isn’t this fun?’” Petroff said. “And she was completely serious. Everyone you meet out there teaches you something and makes you think about your contributions to other people. For Karen and Jim, it’s a positive attitude and joy, and just living the experience.”