Long-distance running, and endurance sports in general, are getting more popular every year.
In most parts of modern life - in how much we eat, sleep, work and play - moderation is the mantra.
And in recent years, that mantra has extended to exercise. Science is increasingly telling us that we need to work out only a few hours a week to fight heart disease and cancer and ageing in general. For instance, researchers say, just 16 or 24 kilometres of easy running could add years to our lives.
But don't try to tell Joanna Reuland that running 16km a week is all she needs.
"I've always got my shoes with me in case there's time for a short run," said Reuland, 25. "It's not something that I have to do - it's something that I love to do."
Lately, she's been loving it a lot - as in, 160km a week. She runs a couple of hours on the way from her home in San Francisco to her office, and often another few kilometres later in the day. It's not unusual for her to take phone calls for work while out on a run.
Ultra-runners like Reuland are unusual, but these days, they're hardly rare with long-distance running, and endurance sports in general, are getting more popular every year.
And this increase is despite the growing clamour of doubters - both scientific and not - who suggest there may not be a lot of health benefits to so-called extreme levels of running. Sure, for years the couch potatoes and others who like to pooh-pooh exercise have insisted that long-distance running can't be healthy - it's not good for the knees, they say, and people can drop dead from that kind of physical abuse.
Their doubts may sound more like excuses than scientific reasoning, but some of their points are backed by fact. People do die while running, after all, although such incidents are rare. And it's true that many recreational runners suffer injuries, often because they push themselves too hard, too soon, sports doctors say.
"Injuries can ultimately undo the whole exercise process. You get injured and it sets you back and next thing you know, you're back at ground zero," said Dr Matt DeVane, a prominent Walnut Creek, California, cardiologist. DeVane said he doesn't discourage people from marathon-distance levels of training, but he doesn't promote that much running either.
"You don't need to do that much to be heart healthy," he said. "I'm usually just trying to get people to move at all."
Dr Todd Weitzenberg, chief of sports medicine with Kaiser Permanente in California, notes that there's a vast field of research proving that running of any kind is healthful. Running has been associated with lower rates of heart disease, diabetes, cancer and ageing-associated disability. Runners live longer than their non-running friends and family.
"You can argue that endurance sports put undue stress on our bodies," he said. "But we're almost genetically hardwired to push ourselves to the limits."
Certainly no one is seriously arguing, he added, that the average marathoner is less healthy than the average couch potato.
What's sparked new debate is recent evidence that a lot of running - a lifetime of running marathons, for example, or the 80-plus kilometres a week demanded by ultra-marathon training - may start to undo some of the benefits of exercise.
Two studies published this year got widespread attention for citing potential negative health effects from exercise - one of them, from long-distance running specifically. One study found that a small percentage of people may have an adverse reaction to exercise that actually increases their risk of heart disease and diabetes. A second study analysed some of the changes to athletes' hearts that could put them at risk.
Yet another study found that, over the course of a year, there were fewer deaths among people who ran up to about 32km a week, and at a fairly leisurely 10-minute-per-1.61km pace, than among people who didn't run at all. That wasn't a shock, of course. What was surprising was that there were fewer deaths among those runners than among people who ran more than 40.22km a week or at a faster pace.
In fact, the longer, or faster, that people ran, the fewer health benefits they seemed to get, until, the study authors joked, a runner could potentially run himself back to couch potato.
That study and others all build evidence for the U-shaped curve of exercise benefits: that greater effort begets greater benefits, but only to a certain point. Then the benefits drop off, suggesting that there's probably a maximum amount of exercise before it starts to become unhealthy.
In another way to look at the U-curve, someone running about 16km a week may actually be healthier than someone running about 96km a week.
While the U-curve is pretty widely accepted, there's disagreement over how much exercise may be too much. Many scientists say, assuming there exists a maximum safe amount of running, it's probably so high that very few runners will ever get there.
"There are probably levels of exercise that are excessive for the average individual," said Dr Anthony Luke, medical director of the San Francisco Marathon. "But we can't arbitrarily say too much is 50 miles (80.45 km) a week, or 100 miles (160.9 km). I don't think everyone has the same limits."
The concern, according to Luke and other sports-medicine experts, is that there does seem to be some small percentage of runners who are at risk for fatal heart attacks when they run at a certain distance or intensity. It's a rare thing - for every 100,000 runners who compete in marathons every year, only one dies - but "no one should be dying at a marathon," Luke said.
But rather than setting safe limits for running, the better approach, many running advocates and sports doctors say, is finding out why those people die, and how their deaths could be prevented.
A middle-aged or older person who's taking up running for the first time should get checked out by a doctor and screened for possible heart conditions.
Plus, doctors and scientists don't yet fully understand exactly what happens to athletes' hearts when they run long distances or at a sustained hard pace. A recent study by the Mayo Clinic, where researchers took care to note the various and many positive health effects from exercise, analysed changes to the heart and blood vessels that are sometimes found in endurance athletes.
The Mayo Clinic scientists found occurrences of irregular heartbeats, enlarged hearts and thickened artery walls, but it was unclear how many, if any, of those effects were potentially harmful or whether they were pre-existing conditions. And it was also not known how many endurance athletes actually experience those effects.
"Any blanket statement about these runners is sure to have exceptions to it," said Dr James Fries, a professor of immunology and rheumatology at Stanford who has studied runners for decades. "We study the mean, and we report what happens to this group or that group. But nobody's actually the average."