His wife, elite rower Kim Brennan, won gold in Rio on Saturday. 

"That's her big moment and she's trained for a decade for this, and that's awesome on one side of it," Mr Brennan said.

"But also, it's a little bit sad as well, to realise that I'm getting a bit older.

"I really miss being able to row, and if I had my way I definitely would've still been there. "

Not too long ago, it seemed Mr Brennan would compete at his fourth consecutive Olympics.

He'd been putting his all into rehabilitation, working part-time and training until he was "fitter and stronger" than ever in late 2014.

 But by early 2015 it was clear heading to the Rio games with the chronic back injury was not viable. 

"It was coupled with the frustration of knowing that my capacity had really improved in the years since London and even Beijing," the 33-year-old said.  

Luckily for Mr Brennan, dedication and a gruelling schedule meant he retired from his professional rowing career a qualified doctor in Canberra.

Mr Brennan, who was born in Hobart, was inspired to study medicine after he was struck down by illness at 16. 

"I never really wanted to be a doctor when I was young, I always had more of an interest in engineering," Mr Brennan said.

"[But] I thought I'd pay my dues back to the medical profession."

But Mr Brennan had become adamant he would compete at the Olympics aged just 9, and after developing a passion for rowing at 12, he wasn't going to give up his dream. 

He juggled completing his medical studies at UTAS in Hobart with rowing. 

"I never really saw myself as an athlete and for a long time I didn't really see myself as properly doing medicine either, because I was always off racing and training and travelling around the world,” Mr Brennan said. 

"It was really, really tough because it's difficult, I think, to do things like this and feel like you're not always giving 100 per cent to one or the other but the reality is you can't."

Now his professional rowing days are behind him, Mr Brennan is grateful he had a back-up plan - but unsure he would be able to live through the stress of juggling academics and athletics again. 

"Now I think of it, I don't know what the hell I'd be doing," Mr Brennan laughed. 

"It's kind of worth all the sleepless nights and a lot of the stress," he said.  

"Looking back on it, I probably wouldn't want to go through that again."

Despite the hardship, Mr Brennan made it to the 2004 Athens, 2008 Beijing and 2012 London games. 

Winning the gold medal in the men’s double sculls at Beijing was the highlight of his rowing career, and the almost indescribable thrill remains vivid. 

"You're kind of ripped apart by all this overwhelming emotion," Mr Brennan said.

"There's joy, but there's also relief, and all of these other emotions that because of the four year cycle and the rarity of it are just amplified ten fold.

“It was almost a terrifying sensation, just how powerful all those emotions are, but also incredible."

Mr Brennan acknowledged the experience was "unreal". 

"Life can feel a little dull in comparison to that memory, so in many ways I look back at that and it's almost like it happened to somebody else," Mr Brennan said. 

"You have to be careful you don't get too caught up in it."

Mr Brennan said although the Beijing Olympics were "rigid" compared to their predecessor in "festive" and "welcoming" Athens, his win defined his 2008 experience. 

"But obviously having a gold medal meant that I had a lot of fun anyway, the Beijing games were great," Mr Brennan said. 

He said London was “pretty unpleasant” due to disappointment and injury. 

After retirement, Mr Brennan's injury continues to prevent him from rowing.

He can do most other exercise, but can only row for two minutes before back pain strikes. 

"I keep trying, every day just hoping that one day it'll be okay," Mr Brennan said.

Mr Brennan's rowing partner David Crawshay remains like a brother to him. 

"We get along really well, he was the best man at my wedding last year and he's a great friend," Mr Brennan said. 

"We've had some rough times together as well as some absolutely amazing ones, so that's a pretty strong bond."

Now he's left rowing behind, Mr Brennan finds some of the discipline learned during his rowing days handy while working.

 "Some of the procedural stuff is really high pressure and using the skills from rowing in terms of being able to just focus on the task ... that's been really helpful," Mr Brennan said. 

"[Rowing] is just a sport at the end of the day. 

“It is important to have something to move onto afterwards that gives you meaning and satisfaction in your life.”