Geraint Thomas wore a giant dragon onesie at Bristol airport on the day he began to realise his life had changed. Such an outfit is not that outlandish for a man as humorous as the Welsh cyclist and it is also obvious that two milestones occurred this month to underpin Thomas’s conviction that he has just entered a new stage of his life.

His first book – the witty and engaging The World of Cycling According to G – has just been published. The wearing of a giant dragon onesie in public, however, is linked to a far more significant moment for Thomas as he married Sara in early October. “We had the stag do months ago,” he says. “It was straight after the Tour [de France] and 25 of us went to Berlin. It was mental – a mix of my cycling mates and Cardiff friends I go on the lash with. My non-cycling mates are proper drinkers with beer bellies.

“We started on the Tuesday night by going out for some food in Cardiff – and ended up having these things called Suicide Wings. Combine that with all the beer and the next morning, on the way to Berlin, I wasn’t feeling great at Bristol airport. I looked a right state and I was wearing the big onesie dragon thing.”

Thomas grins sheepishly. “I couldn’t believe people still recognised me. I was massively hung over and dressed up as a dragon and they were congratulating me for the Tour de France.”

He might tell some amusing stories but Thomas is at his most interesting when he hunkers down in the saddle and allows all his grit and zeal for cycling to emerge. This is a man, after all, who rode much of the 2013 Tour de France with a fractured pelvis. He was in such pain he had to be lifted on and off his bike before and after every day of excruciating racing. Thomas still helped his Team Sky leader, Chris Froome, win his first Tour de France.

This year he was the key ally as Froome won his second Tour – and after 17 stages Thomas was in fourth place. In the end, after a tough stage 19, where he finished 22 minutes behind the winner that day, Vincenzo Nibali, Thomas completed the Tour in 15th place. But that strong start, combined with winning the Tour of the Algarve and his first victory in a cobbled Classic, the E3 Harelbeke in Belgium, offered proof that Thomas could eventually challenge the leading general classification riders such as Froome, Alberto Contador, Nairo Quintana and Nibali.

“I thought I might be up there at the thick end for a day or so,” Thomas says, “but to be there for so long was awesome. From day one I was never worrying about myself. I was always thinking about Froomey. So it definitely gave me the confidence that if I focus on it 100% as a back-up GC rider, behind Froomey, and be protected myself, I’ll have a real good goal for next year.”

Does Thomas believe that, in a few years, he could win the Tour de France himself? “Yeah. I certainly think it’s a lot more achievable than I did three months ago. If I keep on improving, commit to it totally, have a team around me, a podium is certainly realistic. And you’d aim to win it. You would be there or thereabouts – especially if the route was good for me. It’s certainly exciting.”

Would Team Sky eventually allow Thomas to race against Froome – or will there always be one definitive leader? “I think you can have both – but it will depend on the buildup. We’re both honest enough to know that if the other guy is better we’d commit to each other. I certainly would and I’m pretty sure Chris would do as well. But it’s a tough one. It’s like the last Olympics and in the sprint you had [Chris] Hoy and [Jason] Kenny chasing one spot.”

At least there is none of the prickliness that blighted Team Sky in 2012 when Froome and Bradley Wiggins were such uncomfortable team-mates. “Yeah,” Thomas agrees, “there’s none of that. This Tour was good fun. Me, Luke [Rowe], [Ian] Stannard, Pete [Kennaugh] all grew up racing together. It makes a huge difference to be doing the Tour with your mates. Me and Luke used to train on Thursday evenings at the Maindy track [in Cardiff]. It’s surreal to be in the Tour with him, and winning it with Froomey.”

At 29, Thomas is approaching his peak and his appetite for a team leader’s role is growing. “Yeah, for sure. But I can’t see Froomey disappearing any time soon. My deal with Sky expires next year but at the moment I don’t want to leave for another team and be their leader because Sky is the best place to learn – and be in the best position to get a result. Maybe two years down the line I will feel: ‘I know exactly what I’m doing now, and exactly what it takes.’ If I have to leave then, then I would.”

He has compared Dave Brailsford, the head of Team Sky, to “your best mate’s dad”. But, as the racing becomes more serious, can they still talk with blunt honesty? “I feel like I can. At the Vuelta [Tour of Spain] we had a few good conversations about the future. I’m quite comfortable going to him about anything. Obviously he’s my boss but we’ve got that bond which will always be there.”

Brailsford and Shane Sutton identified Thomas privately as the most likely first British winner of the Tour de France a year before Team Sky even began – when he had just won Olympic gold as part of the 2008 track team-pursuit with Wiggins, Ed Clancy and Paul Manning. “I didn’t believe it,” Thomas remembers. “I went ‘Pffffff.’ Then I saw Brad do it [win the Tour in 2012] and that flipped the switch in me. Maybe Shane and Dave saw the same characteristics in me.”

Thomas considers Wiggins’s commitment in 2012. “It’s the most impressive thing I’ve ever seen. It was like the coaches put together a computer programme which Brad followed to perfection. Eat this, pedal this fast for this long, sleep for this long. Brad was like a monk and that year he won everything: Paris-Nice, Romandie, the Dauphiné, the Tour, the Olympics.”

Does Wiggins talk to Thomas about him emulating such success? “Brad talks to me a bit – but he is hard to get hold of. Even with the wedding, just getting a reply from him was tough. But then he got back to me and of course he was coming. I’ve always looked up to him, but he’s still a mate. It’s a weird one.”

Thomas is also close to Froome and, again, he takes inspiration from the way his team leader transformed himself. “Chris is much more assertive now. Before – with all respect – he was a bit like the team joke. You could tell he was special because he had this really good engine. But he was so up and down and his knowledge of the sport was nothing. Even when Nibali went to Astana he was like: ‘Who’s that new Astana guy?’ He’s a lot more into it now. Certainly with his rivals, he always knows what Nibali’s doing or what he’s said. That’s the most impressive thing. He’s so aware of the media, and reads it all, but none of it gets to him.”

Was Froome taken aback by the vitriol shown towards him during the Tour when he was spat at and had urine thrown at him? “2013 was the biggest shock. This year he just brushed it off. But yeah, when we were having dinner, he certainly got more animated with us than he did in front of the cameras. When Laurent Jalabert [the 1995 winner of the Vuelta] said Chris was from another planet that fuelled the fire for the hatred.

“But why does no one give Nibali and Astana an even harder time? Why is Contador never questioned like Froomey? It baffles me. Chris won one big stage on the Tour but other than that day he was just defending the jersey. He didn’t really light up the race. There were no doubts over the way Quintana went in the last week. People just said: ‘Oh, he’s just a really good climber.’ But Froomey? ‘Oh, he must be cheating …’”

Thomas denies any suggestion that there is still widespread doping in cycling today. “I am sure some people will always try to find shortcuts – but the fact I’m competitive shows how much cleaner it is.

“Percentage-wise it’s tough to know [how many riders are doping] but the majority of teams are doing it the right way. It’s good I’m being tested more and more. Even on the stag-do in Berlin I had to fill out [a form about] what hostel we stayed at. But I’m glad the testers didn’t turn up. I don’t think the boys would’ve been too happy if there was a knock on the door at 6am.”

On the more serious issue of whether Team Sky will release further data in an effort to prove they are racing clean, Thomas says: “It could help but a lot of the haters will always find a reason even if you give them all your blood results and power files. Why is it only Sky who have to do this? You don’t see Astana or Katusha giving out all their power files. Maybe Froomey and the team are too polite.”

Thomas is the best antidote to accusations that Team Sky is a robotic machine. His book, which revels in the sport’s grand traditions, confirms his love of racing in Belgium and Italy, and devotes chapters to cycling perennials such as “Cafés”, “Rain” and “Soigneurs”. It is a warm insight into the kind of bike racing which has consumed Thomas since he was a kid begging his dad to take out a Eurosport subscription so he could watch every Classic and the Tour de France after school.

Team Sky’s Geraint Thomas protects his team-mate Chris Froome’s lead in the Tour de France. Photograph: Benoit Tessier/Reuters

He concedes that, in contrast: “Sky, as a company, is not the friendliest. They’ve definitely got their enemies and people didn’t like the way they came on the scene with Dave saying we’re going to win the Tour with a British guy in five years. They thought: ‘Who do they think they are – thinking they’re better than us, wanting to change everything?’ But everyone wears skin-suits now, and filled-in helmets, and does warm-downs.”

Thomas, unlike Team Sky, is happy to be compared to a penguin. But David Millar, as a father of young children, thought of the film Madagascar when he said of Thomas: “Cuddly on the outside – the spirit of a hired assassin within.”

“I didn’t even know what Madagascar was,” Thomas grins. “But, when you’re racing, the steel comes out. I haven’t had the full-on road years like some guys so I have a lot more time in me to show that inner spirit.”

His smile broadens when he is asked what he might do if, one day, he does actually win the Tour de France? “That would be some party. I’d get a very special onesie out for that one. The Tour gives you real purpose and it excites you. I’ve got that target now.”

The World of Cycling According To G is published by Quercus Books and is available from Guardian Books