The reality struck recently during the eight-hour journey back to Spokane from the San Juan Islands.
I drove alone in the minivan that's hauled our family on countless expeditions, the nature of which was revealed by the bikes, skis, canoes, fishing rods, kayaks or backpacks fastened on the roof rack.
This time the van, minus its rear seats, was jampacked with the belongings of oldest daughter Brook, who's transitioning again between life adventures.
I found myself mentally detouring down the inevitable route of pride and anxiety that comes with being a father who's infected his offspring with a love for the great outdoors.
We hiked and kayaked on this trip. In the Landers family, holidays and other special occasions usually have some connection to being outside.
Youngest daughter, Hillary, checked out a potential eighth-grade boyfriend a decade ago by leading him on a 25-mile bike ride. He didn't pass the test.
Brook's first six birthdays were celebrated with trout fishing and a campfire meticulously poked and tended by her grandfather.
Potty training didn't come naturally to Brook, but she caught on quickly when we put her potty chair under a beautiful cedar and told her it was the throne for the fairy queen.
Hillary was just 7 when she assumed the bow position in my boat for a five-day voyage on the Green River in Canyonlands National Park, Utah.
She'd been a lily-dipper most of the trip until a thunderstorm came from nowhere over the canyon rim and smacked us with thunder and 40 mph gusts.
"Paddle hard, Hillary," I yelled over the roar of wind. "Daddy needs your help."
Not only did she hold her own in the bow, she also buoyed our spirits as we weathered the storm, clinging to roots along a slick rock cliff.
With a stream of water pouring off her rain jacket, she sang songs while our family hunkered in partially swamped canoes between a cliff and whitecaps a half-mile from anyplace to pull out and miles from any help.
I wasn't surprised that Hillary worked in college as a sea-kayaking leader in the San Juan Islands. But does she know enough about tide charts, hypothermia and rescue? I've shivered thinking about it.
She tells me of taking her groups foraging to make meals of seaweed and other wild edibles, and I wonder if I fully explained the consequences of eating poisonous plants.
Should I remind her the ancient Greeks used water hemlock for executions; that the plant killed Socrates?
Should I be sleepless or proud that my daughter knows how to prepare, eat and enjoy stinging nettles?
Hillary, a daydreamer, once drifted off a bike trail and tumbled over the ledge of the cliff.
She was hanging from the brush when I got to her, holding the bike with her feet to keep it from going farther downhill.
She'll never let me forget that I was more concerned at first with rescuing the bike.
Now Hillary has a plan to bicycle across the United States this fall. As I tuned her bike's brakes, I hinted that job experience might be beneficial at this stage of her life.
"Weren't you just out of college when you rode your bike across the country?" she said.
I wasn't surprised that during a year of Spanish study in Spain, Brook hiked the Camino de Santiago de Compostela pilgrimage route in the Pyrenees and toured the country on a bike.
Brook is leaving to lead a school group to Peru and I'm wondering if I emphasized the importance of hydration and acclimatization and the consequences of high-altitude sickness and drinking unpurified water.
We've celebrated New Year's for 18 years by skiing 6 miles into an unplumbed, wood-heated backcountry cabin in the Methow Valley.
But I trembled when Hillary bought a used Randonee ski package, especially when she announced she was headed to the top of Mount Baker.
Have I emphasized the dangers of avalanches and glacier travel over snow-veiled crevasses? Should I tell her that I backed away from my overexposure to those hazards after she was born?
"It's dangerous to open the doors on my Subaru," Brook said on the phone as she left the San Juans after three years of teaching high school Spanish.
Her next stop is teaching Spanish in the United Arab Emirates. I don't know much about living in a desert, so how could she?
"Just don't break any laws," I said, mustering the only advice I could think of. "The police might chop off your hand."
She rolled her eyes, searching for an escape in the depths of her cranium as I began planning a way to share the burden of worrying.
Next year I'm letting my wife drive home eight hours alone after spending a weekend outing with her daughters.
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