No longer do we have to sprint marathons while training for ultramarathons, take up Tough Mudder challenges, pose for six-hour hot yoga classes, or find an extra 90 minutes every day to sculpt our upper arms. Or even think about such things. For - hurrah! - a new fitness fashion is coming in: moderation.

It started in Hollywood, of course. Former queen of looking lean, Jennifer Aniston, has confessed that she has let her intense exercise regimen slip a little - on purpose. Previously, the 45-year-old admits, she worked out "almost every day", doing "40 minutes of cardio: spinning, running, the elliptical, or a combination of all three", followed by weights and Pilates.

But she's "eased up in the last couple of years", and now recommends that women "take some time off and don't do anything and enjoy your life".

Singer Gwen Stefani agrees: "This past year, I kind of stopped working out," the 44-year-old said recently. "I think my body just needed a break. And so I did that and focused more on feeling good as opposed to beating myself up."

Meanwhile, the newer generation of female stars have never been that fussed about acquiring the muscle length or definition of your average Olympic gold medallist. Model of the moment Cara Delevingne goes to the gym only twice a week, apparently. TV's sweetheart Holly Willoughby has said of her exercise routine: "I have great intentions, but never seem to keep it up."

Even Coldplay star Chris Martin appears to be on trend after parting with gym maniac Gwynnie and starting his rumoured relationship with Jennifer Lawrence. His new girlfriend is reportedly no fan of exercise, working out only for her role in Hunger Games - mostly through skipping sessions.

Fitness expert Sophie Mathews, a former professional windsurfer and kiteboarder, thinks we're re-discovering the fundamental purpose of exercise: "Working out should benefit body and soul. It should be about overall wellbeing, not just how you look.

"But we've seen exercise become so competitive - especially among women - fuelled partly by the rise of Instagram and fitness blogs. People put together ridiculous fitness routines and then post pictures of themselves, which say, 'Look what it's doing for me, can you do it?'?" Think actress Vanessa Hudgens showing off her aerial yoga moves.

Sophie admits she's been caught up in extreme fitness fads, such as the Insanity workout, which replaces traditional moderate-intensity exercise with maximum-intensity exercise, and trades short intervals of intensity for short periods of rest. Or CrossFit, which mixes gymnastics with strength and endurance work. "I'm very fit," she admits, "but I found some of these workouts too much. The secret has to be finding exercise that you enjoy but that is sustainable long term."

James Conci-Mitchell, a fitness trainer at the six3nine gym in Covent Garden, London, agrees. "Moderation and sustainability are what we promote and what clients want. We encourage working over the long term - making positive lifetime changes, not 12-week extreme transformations - and it seems to be more popular."

James thinks the new attitude is partly driven by going through the recession. "Before, everyone wanted a quick fix. Now they are a lot more wary. They want value, not fads."

We're also becoming smarter about time, says Sloan Sheridan Williams, a personal trainer and life coach based in central London. "Exercise should be something you can fit into your life, but these huge challenges - like 24-hour canoe rides or ultramarathons - can take it over. It's not just about the event itself, but the months of dedicated training you need to do. Not to mention time out with injuries. I think people are increasingly looking for ways to exercise that they can share with family instead."

Pilates teacher Lynne Robinson worries: "Do we even really need to be super, super fit, especially if this extreme fitness comes at the expense of less time with friends and family? It's worth considering, also, that we now know that overdoing exercise isn't good for our immune system or joints."

Indeed, some of the backlash against extreme fitness is probably due to the high casualty rates. Tim Allardyce, a sports physiotherapist in Croydon, reports seeing many more clients with injuries from ultramarathons and CrossFit sessions. "People have been going to more extreme levels and then getting injured more readily. They have been pushing their bodies beyond their limits without adequate training, or proper fitness levels."

Sloan Sheridan Williams also thinks we're getting wiser to the effects of physical burnout. "Hard workouts or training sessions used to be seen as an antidote to office stress, but now we realise that they are simply adding to the body's burden. Instead, people are becoming better informed about types of sport that can be beneficial, whether that means walking the dog, trying hatha yoga [which uses gentle stretches], or t'ai chi, low-intensity swimming or cycling."

There's no doubt that some are maintaining intensity - although for much shorter periods of workout. Hugely popular is High-intensity Intermittent Training, which involves very short bursts of high-energy exercise, and has been scientifically shown to be as good for the heart as longer, sustained workouts. Actress Helen Mirren also likes a fast fix: in July, she revealed that her fitness routine consisted of 12 minutes of daily Royal Canadian Air Force exercises in preference to visiting a gym.

Meanwhile, in New York, the Wall Street Journal has reported that bankers are now opting for running mile-long races instead of marathons, their enthusiasm supported by research published in the Journal of the American College of Cardiology in July that said milers enjoy the same mortality-fighting benefits as marathoners.

Best of all, science is rewarding fans of what we could call the slow exercise movement with studies that suggest running something like the Marathon des Sables - the annual six-day, 156-mile ultramarathon across the Sahara - is not necessarily a "get-out-of-jail free" card for your overall health anyway. Some reports suggest that just standing up (at your desk) is pretty darn healthy. Standing, it seems, can reduce upper back and neck pain and improve moods, according to a Minnesotan study in 2011. And James Levine of the Mayo Clinic, the medical research centre in Minnesota, reported in 2012 that standing at your desk was healthier than going to the gym, after a day spent sitting down.

Californian biomechanist Katy Bowman has even coined a phrase for those who work out like mad, only to collapse, albeit smugly, in a heap. "?'Actively sedentary' is a new category of people who are fit for one hour but sitting around the rest of the day," Bowman has said. "You can't offset 10 hours of stillness with one hour of exercise."

She adds: "Walking is a superfood. It's the defining movement of a human. It's a lot easier to get movement than it is to get exercise."

Lynne Robinson agrees: "One of the reasons Pilates is popular is that it's intelligent exercise, meaning that the precision with which you do the exercises makes it extremely effective. It's quality of movement, not quantity, that counts."

The Telegraph, London