The columnist for Runner's World magazine runs five or six days a week, and he always starts with a "trial mile." If stiffness and soreness persist beyond the first few minutes, he quits and walks home.
"That first mile seems to be the truth serum," he said. "I almost always go my usual amount. But I'm willing to call a halt.
"I'll tell you: That has saved a lot of problems."
Henderson turned 58 recently. He's been a runner for 43 years. Many people his age have either quit running or moved indoors to the cushioned surface of a treadmill.
The ones who keep going eventually run into inescapable realities: slower times, less forgiving joints.
But that's no reason to rule out running as a lifetime activity, Lincoln, Neb., physical therapist Jayne Snyder says.
Snyder, who is past 50, has run for 22 years. She has finished more than a dozen marathons, including one over Memorial Day weekend in Traverse City, Mich.
She used to run 45 to 50 miles a week. Now she usually tops out at 35. Her training pace has slowed from nine-minutes miles to 10 or 11.
She and her training buddies in Lincoln dub themselves "The Turtles." They sometimes go on 20-mile runs during marathon training.
"We have this slogan: 'Speed kills,'" she said. "A lot of fast runners don't keep running long term."
Aging tends to diminish not only speed but also strength and flexibility. Runners are especially prone to inflexibility because they work the same muscles over and over again.
Snyder, vice president of the American Physical Therapy Association, recommends stretching for five to 10 minutes a day. Good times to stretch are after a run or at bedtime, when the muscles are warm.
Focus on loosening the hamstring and calf muscles on the back of the legs. Snyder said tight hamstrings can shorten your stride and overwork the quadriceps muscles on the front thighs, leading to kneecap pain. Tight calves can lead to plantar fasciitis in the foot.
While most runners battle leg and foot injuries at times in their careers, there is no evidence that running is inherently destructive to the skeletal system. Stanford researchers spent eight years tracking 451 runners in their 50s, 60s and 70s. The runners had been at their sport an average of 12 years when the study began.
They were several times less likely than 330 non-runners to develop a disability during the eight-year study. They also were less likely to die prematurely.
Snyder believes virtually all running injuries are preventable. Besides tight muscles, she said, common causes are wearing the wrong shoes and increasing mileage too fast.
Boost mileage by no more than 10 percent per week, or 5 percent if you're coming off an injury, she said.
If you run four or five days a week, rotate at least two pairs of shoes, she said. And use different models.
"If I use the same kind of shoe every day, the impact through my lower extremities is going to be the same," she said. "If you have two different models, you're going to vary the weight-bearing a little. So overuse injuries don't occur quite as often."
Not all older runners become turtles. Ronn Baker of Omaha, who turns 62 this month, is faster than many runners half his age. He won his age group at the Bolder Boulder, a Colorado road race over Memorial Day weekend that attracts some 45,000 entrants a year. Baker covered 10 kilometers in 41 minutes. He ran a 34:57 at age 55.
Baker does make concessions to age. He stretches for a half-hour each day and takes two days off a week from running. His off-days usually feature swimming or light weightlifting.
He likes cycling, too, but has no interest in triathlons.
"When I'm doing my cross-training, I try not to be as intense," he said. "It's really a rest day for me.
"I think cross-training should be mandatory for any runner who's over 40. I know that not all serious runners cross-train. But I think it will prolong your career."
Cross-training isn't for Jay Dirksen, the 56-year-old cross-country coach at the University of Nebraska. He runs seven days a week.
"That has a lot to do with my personality," he said. "Certain things I do every day, like reading the Bible. It's better for me to have a routine, and I'll get it done."
Dirksen has run 45,000 miles since high school. He used to run 100 miles a week. Now he runs 20.
"I just try to run the way I feel," he said. "Some days I don't go very fast. Other days I do.
"You get yourself in trouble when you tie yourself down to doing something when you don't feel good that day."
Dirksen has bounced back from knee surgery last summer to repair cartilage damage that he believes was unrelated to running.
Henderson, the Runner's World columnist, missed about a month of running after heel surgery 30 years ago. No other running injury has sidelined him for more than a few days.
But he has learned to listen to his body. An early champion of "LSD" Long, Slow Distance Henderson figured out that running for longer than an hour aggravates the calf and heel problems he developed in high school.
"There's only so much scar tissue you can build up before it starts to be a problem," he said.
One way to minimize injury risk, he said, is to schedule just one hard training day a week. The hard day can be either speed work or a long run.
Most days he runs 30 minutes. And he consciously starts slowly. His "trial mile" might last 10 or 11 minutes, he said. He covers the second mile in nine or 10 minutes, and the third in eight or nine.
Starting slowly is a good idea at any age or ability level, he said. Even Boston Marathon champions do it. "I see a lot of them leaving their hotel going 10-minute miles."
Henderson still runs marathons on occasion, but he sprinkles longer runs with one-minute walking breaks.
"I wouldn't be running or finishing marathons without walk breaks," he said. "They're leg savers."
Henderson lives in Eugene, Ore., a city overrun with people fleet of feet. Olympians live there to train, and the University of Oregon is a perennial power in track and field.
Those collegians have lived only half as long as Henderson has run. He said it doesn't bother him when they whiz past him on a trail.
"There are people who run much faster than me, but also people who run much slower," he said. "I slide backwards as the years go by, but that's OK.
"I've seen a lot of hotshots come and go."
Stretching your running career
Hamstring stretch: Lie in a doorway with one leg on the floor and the other against the wall. Hold that stretch for several minutes. Then stretch the other leg.
Calf stretch: Lean against a wall with your palms, with one foot 12 to 18 inches from the wall with the knee bent. Try to apply steady downward pressure on the back heel so it touches the floor.
Lower back stretch: Drop into a catcher's stance, with your lower back curved, your neck relaxed and your hands on the floor. Lower your chin to your chest
Shop for running shoes, clothes and much more at the Active Sports Mecca's Running store
Check out The Ultimate Test, Active.com's guide to all things marathon
Get ready for your first (or next) marathon with our free ActiveTrainer programs