It may well be something you've heard a million times before. But when it comes to your fitness, it's well worth remembering.
If instant gratification is your thing, exercise can be a drag. It's not something you do once, then sit back and reap a lifetime of rewards.
Those rewards only come with hard work, consistency and self-discipline.
"You're only as good as your last training session," said sports scientist Tony Boutagy.
"In other words, you only get health benefits from a session for up to about 48 hours afterwards."
And no-one is immune to this deconditioning effect.
Whether you are a marathon runner, a regular gym-goer or someone who only walks a few times a week, you'll likely lose half your fitness if you don't train at all for a week, said Nigel Stepto, associate professor in exercise physiology at Victoria University.
"Of course the marathon runner's fitness would still be greater than someone whose main exercise is walking — they were fitter in the first place," he said.
"But after a week of no training, both would be half as fit as they were the last time they worked out."
Research does suggest, however, that those with higher levels of fitness who have been exercising for longer hold onto their fitness for longer than newer exercisers.
What is fitness?
Your overall fitness level can be broken up into three components:
Cardiovascular fitness — your body's ability to transport and utilise oxygen. This includes activities like walking, running, cycling, swimming and aerobics.
Balance and flexibility improve the full range of motion of your muscles and joints. Yoga, tai chi and pilates can all help you achieve this.
Musculoskeletal fitness helps strengthen muscles, improve bone density, maintain a strong core and help you maintain a healthy weight.
All three forms of exercise are necessary for a strong, durable and healthy body.
When you have a break from exercise, the most noticeable change will be to your cardio, or aerobic, fitness.
This is the ability of your lungs, heart, and blood vessels to take in oxygen and transport it to your muscles, where it is used to produce energy for movement. The more you work this cardio-respiratory system, the more efficient it is at doing its job.
But take a break of more than a couple of days and your cardiovascular fitness drops off very quickly.
You've probably noticed you get puffed when running up the stairs after missing a few workouts.
That's because when you lose cardio-fitness your heart has to work harder to get enough blood and oxygen to your deconditioned muscles, which require extra fuel because they have had a reduction in some of the important proteins in the energy-producing pathways and components of the muscle tissue, Dr Stepto said.
This results in a higher demand for blood to bring oxygen and remove carbon dioxide and other undesirable by-products of energy production while performing the exact same exercising session you did before having the prolonged break.
What else do you lose? Muscle strength.
"You lose the cross-sectional surface area of your type two muscle fibres," Dr Boutagy said.
"Muscle fibres have a genetic size to them. If you go below that size, they self-destruct and turn into connective tissue."
Sure, missing a workout here and there isn't a huge deal. After all, we have so many competing factors in life pulling us in different directions. Some days exercise isn't going to happen.
But, "within two weeks you'll probably start to show a 7-10 per cent loss in strength levels," Dr Boutagy said.
"Most data suggests that you'll lose at least 70 per cent of the adaptation you've built through exercise after about three months."
But Boutagy stresses many of these studies focused on people who became totally immobile.
Research has found for those who maintain some movement and light exercise, the loss of muscle strength is a bit slower.
Other studies have found the muscle fibres of athletes — such as runners or rowers — tend not to change as quickly. Although certain sport-specific muscle strength can start to disappear within a few weeks.
Truth is, most of us aren't athletes, and struggle to get the minimum recommended amount of exercise each week. So it won't take long to see any hard-won muscle strength disappear.
Building back up
Enough of the bad news. How long does it take to re-build your cardiovascular fitness and strength?
"The further you are from your fitness potential (read: the more unfit you are), the more profound your results will be in the first 12 weeks," Dr Boutagy said.
"Studies are unanimous that beginners can pretty much double their strength within this time."
In the fitness industry this is known as "newbie gains" and it's an exciting time.
"In terms of cardio, most studies show that within three to four weeks you can improve your VO2 [your maximum rate of oxygen consumption] somewhere between 20 and 30 per cent using interval training," Dr Boutagy said.
Having said that, he warns, genetics play a big role in exactly how long it takes to get your fitness back after crawling out of the "frump rut".
If you're starting from scratch or starting again, the best thing to do is to just get moving. Perhaps kick off with regular walks and build from there.
Getting fit and staying fit is a continuous, life-long process that will ebb and flow. "Fitness" will mean different things for you throughout the various stages of life.
The most important thing is to keep moving in some way, preferably every day.
And none of this means you should never take a break. Rest days are important for your body too.
But you need to make sure your rest days are not outnumbering your training days.
Cassie White is a Sydney-based personal trainer, yoga coach and health journalist.