Seven years ago Steve Way weighed 16 1/2 stone and had high-blood pressure, a 20-a-day habit and an addiction to takeaways and chocolate. So, like many other people, he began running to get fit. Only he didn’t stop. And 26,000 miles later – more than the circumference of the globe – and at the grand age of 40, he has been selected to run the marathon for England at next month’s Commonwealth Games.

It is a staggering, unprecedented, and almost fantastical tale. One that Way, who somehow fits in running 130-140 miles a week with a nine-to-five job in a bank, admits is “completely ridiculous”. And that might be understating it.

“I am a one-man band: a self-coached club runner,” he says. “At school I was the guy who hid in the bushes with my fat mate during the first lap of the cross country and then rejoined the field when they came round again. I have never competed in a major championship before and until recently I had almost no contact with British Athletics. And now I will be running at the Commonwealth Games.”

You might call him athletics’ answer to Rocky Balboa, the underdog who got a surprise shot at the big time – except that instead of running up the 72 steps to the Philadelphia Museum of Art he sprints round the duck pond in Poole Park. And rather than punching slabs of beef in a meat-locker he has T-bone steaks with oven chips for tea as a treat.

But Way’s dizzying ascent from club runner to international athlete did not require a scriptwriter. It came about from hard slog, geeky attention to detail, and an under-the-radar performance at this year’s London marathon on 13 April. Having started in a group for sub two-hour-45-minute athletes, a minute behind the elite field containing Mo Farah and the world record holder Wilson Kipsang, he ended up overtaking several established athletes before finishing 15th in two hours, 16 minutes and 27 seconds.

He had beaten his personal best by nearly three minutes, and was the third British athlete home after Farah and Chris Thompson. And, crucially, he finished 33 seconds under the qualifying standard for the Commonwealth Games. Afterwards the stewards attempted to usher him to the tent where the marathon’s biggest stars were having their post-run massages and recovery drinks. “Er, my kit is over that way with everyone else’s,” he told them.

Incredibly – a word that keeps coming to mind when you hear Way tell his story – the London marathon was not his main target. He only decided to compete eight days beforehand. All his training had been focused on breaking the UK 100km record, something he achieved at the British Championships at the start of May. His time of 6:19.19, an average of 6.06 per mile over 62 miles, was 46 minutes clear of his nearest rival.

“Part of me can’t quite believe how well I did in London because of the improvement I made,” he says. “But I also think I didn’t actually give it 100% because I recovered so quickly and my legs didn’t feel like I had put them through much.”

Way’s final marathon preparations were hardly ideal either. While the elite field spent the night before the race at luxury hotels, he slept in a camper van parked on his cousin’s drive to save money. The $1,500 (£896) he earned for finishing in under 2:17 was the most he has ever won from athletics. “My wife, Sarah, is an accountant and likes to keep track of my income and outgoings from running,” he explains. “Last year I just about broke even when you take into account travelling to races.”

So where did this hidden talent come from? It wasn’t evident at school. Or during the next 15 years. Instead Way’s life followed an unexceptional pattern: every so often he tried to lose weight by eating healthily and jogging – and a few weeks later he always gave up.

Yet Way sensed he might be “a little bit different” when he entered his first marathon in 2006 on a whim and, after three week’s training, finished in a highly respectable 3:07.08 despite being “a fat bloke bouncing along next to club runners”. But then he didn’t put on a pair of running shoes for 18 months.

“Towards the end of 2007 I could hardly sleep at night,” he says. “I was coughing and waking up because of the smoking and it was impacting on my wife too. At that point half our meals were takeaways and I would eat chocolate and sweets all the time. I was 16 1/2 stone and realised I had to do something radically different to break the cycle.”

And so he did. He quit smoking and fast-food and began jogging, and before long was following a 24-week plan from the book Advanced Marathoning. He aimed to break three hours at the 2008 London Marathon. Instead he ran it in 2:35. “I came 100th and the concept of finishing in the top 100 was just awesome,” he says. “So at that point I joined an athletics club and started to take it more seriously.”

In 2009 he ran London in 2:25 despite injuring his hamstring the week before. He collapsed when he crossed the line, having torn most of the muscles around his pelvis, and was on crutches for weeks. Yet the thirst for self-improvement only grew. In 2010, helped by advice from the British Olympic marathon runner Liz Yelling, he broke 2:20 for the very first time.

But for the next three years, progress slowed. He tried upping his mileage to 160 miles a week but his body rebelled and his heart rate briefly shot up by 30 beats a minute. He was injured a few times, most frustratingly just before the 2013 London marathon when in “unbelievable” shape. So he decided to try an international 100km race and, having won it by over 40 minutes, returned to the marathon where he smashed his personal best this year.

The key, he thinks, is that he recovers so quickly from long, fast runs. “I don’t seem to put my body under that much stress when I do marathon-pace type efforts,” he explains. “Even doing back-to-back long runs at the weekend, when I might do a marathon on the Saturday and 40 miles on the Sunday my body seems to accept it quite nicely.”

The athlete runs twice a day – either before work or during his lunch break, then again in the evening. He plans to take only one day off from now until the Commonwealth Games. “I really don’t feel any better after a complete non-run day,” he says. “So if I need a rest I just do a six-mile jog.”

Way can tell you about all of his runs since 2007: the distance, the pace, how he felt. It has all been tracked. Even when he runs, he thinks about running. “I am a bit of a running nerd,” he sighs, in a self-deprecating fashion. There is a touch of Hugh Laurie about him, something charming, lovable and slightly eccentric.

So how well could he do in Glasgow? “Obviously you can’t count out a medal because you never know what will happen but realistically, with the high-quality Kenyans in the field, I’d be stupid to think it’s likely,” he says. “But I have no fear about stepping up and I’m aiming to improve my PB and break the British record for a 40-year-old.”

A more realistic medal chance will come at the 100km World Championship in Doha in November. If Way can run under 6:20 again he will be in the mix for a gold medal – something that both staggers and excites him. “I still struggle to see myself as a proper athlete,” he says. “I am just a man who has got quite obsessed with his hobby and can’t put it to rest.

“To be honest, I wasn’t really expecting to move to the next level but I surprised myself. So now I am asking – what else could there be for me?”