"When we're training, we talk triathlon a lot," the former discloses. "Back in the flat, we just talk a lot of rubbish really. Our lives are pretty much about triathlon but we'll sit and try and take our minds off it by playing Fifa or sitting around."
Idle time is at a premium for Scotland's two brightest hopefuls. When not compiling laps in the pool, or miles on their bikes or on the run, there are lectures at Stirling University to attend and assignments to complete. Even then, the pair are inseparable, both in the second year of degrees in mathematics, another field in which their shared mission is to graduate with honours.
Having won silver and bronze at last year's world under-23 championships, the classmates have headed to Yokohama for the latest leg of the ITU's World Series where all three Olympic medallists, the brothers Brownlee included, are in the assembled field. For the rookies, this should be a rite of passage, a mere opportunity to absorb knowledge. Yet in his debut on the elite circuit last month in Auckland, Sheldon finished 12th, a significant statement of intent.
"Last year, I was always targeting world juniors," says the 19-year-old. "Once I achieved a medal there, I went: 'let's get into the seniors and see what I can do there.' Coming third in Alicante at the end of 2013 was a big surprise - that was the first go I'd had at that level. To beat a lot of top athletes was pretty special. But after that, I knew where I was heading. I had a good winter training and then Auckland went well."
Better than most expected. Not, however, for Sheldon, nor his coach Blair Cartmell, who had looked down the list of former junior medallists and noted just how many had quickly advanced into the grown-up mix. With instantaneous returns, there was a temptation, he admits, to shred the old blueprint and begin a full-throttle chase for ranking points and prize money.
Cartmell, once a handy competitor, urged patience and found no dissent. "The Commonwealths are still the big aim, then the world under-23s," Sheldon affirms. "I definitely want to have another go at that title and then next year go for the Series."
With the injured David McNamee pre-selected for Scotland's team for Glasgow, there are two potential individual vacancies to be filled. Ultimately, from among those who have proven their worth following the PruHealth London Triathlon on 31 May, an objective assessment will be made. "I'd love to race the Commonwealths," Sheldon says. "I'm from Hamilton. I was brought up around there and I had my first bike ride around Strathclyde Park. So it's home turf." A positive impression in Japan and he is unlikely to miss out.
Austin, however, has lost ground due to an injury which delayed the start of his challenge. Six months the senior of his friend and foe, he has watched Sheldon go forth and prosper while focusing instead on his own rehabilitation. Sheldon feels his pain, with the roles reversed from 12 months ago when he served his own sentence on the sidelines. "But having Marc to chase when I got back was a big help," he recounts. "In a way, I found it annoying watching him doing what I should potentially have been doing. That drove me to get back to fitness quicker."
Japan also represents the start of the countdown to Rio. The first qualification points towards the 2016 Olympics are on offer and his initial impact has left Sheldon pondering if he could elevate himself into the frame. First things first, though.
"My main target is to fix the mistakes of Auckland. I finished that race happy but also quite frustrated. I'm usually a decent swimmer but I had an awful swim there. So I want to sort out a few of those tactical errors and hopefully get out of the water fast enough to challenge."
Glasgow hopeful Natalie Milne will race the women's event in Yokohama as Jane Egan and Alison Patrick compete in the first-ever Para-Triathlon World Series event.