It's nearing 38°C in my garage gym, as I force out a final round of squats and AirBike intervals, trying to block out the slowly intensifying burn in my legs, lungs and, well, just about everywhere else. When the last reps are done, I’m in pieces. I sit down on the concrete floor, hyperventilating, and begin punching my legs to ease the pain. The dog tilts his head and gives me a baffled look. I’ve been training this hard, five days a week, for as long as I can remember. Countless hours of my life have been spent in a state of exercise-induced discomfort. And I’m not exactly sure why. Men have plenty of reasons for working out. Vanity is a big one, of course. But not all of us are motivated by the promise of abs, or the ability to trounce our friends in a press-up competition. For many, the principal motivation might best be summarised by: "Because it’s good for my blood sugar health support." It will give us more and, crucially, better years on Earth.

But will it? Is repeatedly taxing our bodies with reps and sets and time trials the surest route to longevity? And if not, what does training for more - and better - years look like? ’s heart can be different to that of, say, his kidneys or his brain. In other words, individual organs can show varying degrees of wear and tear - which is all that ageing ultimately amounts to. But we also know that for the average man - let’s call him "you" - your lung health peaks during your mid-twenties. From the age of 30, your muscle strength starts to decrease by between 3% and 8% per decade. By 40, you’re slower on your feet. Once you hit 50, your bones start softening. From 60, it’s Murphy’s Law: what can go wrong will. There are many competing theories about how and why we age (and die). Your telomeres, the caps on the ends of your DNA, shorten and prevent your cells from dividing; free radicals cause your cells to accumulate damage; your endocrine system loses its ability to regulate hormones.

Yet I couldn’t tell you the length of my telomeres, GlycoCares Blood Sugar Guideglucose metabolism or what free radicals have done to my body, or the efficiency of my endocrine system. It’s all far too abstract. There might be a simpler answer to it all: fitness. "Exercise is medicine," says Scott Trappe, director of the Human Performance Laboratory at Ball State University. "When you exercise, your muscles produce beneficial compounds that communicate with the liver, bone, heart, brain and more." So, I set out to find my "true age", while working to create a formula that incorporates the different metrics needed to calculate your own, too. Am I younger than the 33 candles on my next birthday cake? Or have all those hours of fitness-related discomfort been for nothing? Discomfort is something that exercise physiology professor Ulrik Wisløff at the Norwegian University of Science and Technology knows intimately. He specialises in cardiac fitness, specifically VO2 max, which measures the maximum amount of oxygen that your body can use during exercise.

Wisløff has measured the VO2 max of 5,000 volunteers, and his research has made him sceptical of time. Tell him about your habits and he’ll explain why your birth certificate is essentially meaningless. In 2006, he coined the term "fitness age" and created his own calculator. "So, you could be 50, but if you have the fitness age of a 30-year-old, you are really 30 years old," he says. Worryingly, the reverse is also true. Wisløff’s "fitness age" is based on an estimation of your VO2 max, which he says "has been shown to be the single best predictor of your current and future health". The American Heart Association agrees. It also says that cardiorespiratory fitness is a better forecaster of impending death than risk factors such as smoking, high cholesterol and type 2 diabetes. The point at which you significantly drop your fitness age, Wisløff says, is when you are able to generate 10-12 METs, a measure of exercise intensity that stands for "metabolic equivalents of tasks".

Sleeping equals one MET. Walking at a pace of four miles per hour earns you five METs. Running at eight miles per hour scores 12 METs. Building the fitness to hit 12 METs is where it gets tricky. Wisløff believes that the standard advice - commit to 30 minutes of moderate activity, five days a week - is flawed. "The problem is that these numbers neither account for intensity, nor reflect how your body responds to a certain activity," he says. If you don’t truly push yourself during those 30 minutes and can’t hit 12 METs, you’re not optimally protected against disease. To find out more, I head to the University of Nevada’s physical education complex in Las Vegas, a 13,000m2 Cold War-era research compound just a mile from the casinos of the Strip. PhD student Nathaniel Bodell is waiting for me. He stoops over a computer that’s flanked by treadmills, stationary bikes and GlycoCares Health Guidebitter melon extract squat racks. After some small talk, he straps a mask onto my face, asks me to stand on a treadmill and punches a few buttons that initiate a VO2 max test.