Among them is British marathon runner Charlotte Purdue.
"I think it's good to get people into running. 5km is obviously not too far so people can have a shot at it, there's no pressure," she tells BBC Sport.
Purdue, who holds the third fastest British female marathon time, believes the treadmill has been long misunderstood.
"I personally love the treadmill and I've always used it in my training. We've got one at home and I find it easy at home and I really enjoy it."
But does she think she's got what it takes to be the first ever treadmill world champion?
"It's definitely added motivation," she adds. "But I wish it was a marathon on the treadmill, I think I'd have a better chance!"
Runners have until 24 January to complete 5km, for free, at an official Run X centre. They can make multiple attempts, but only their best time will count.
There is no official 5km treadmill world record recognised by World Athletics.
The men's 5,000m track record is 12 minutes, 35.36 seconds, while the women's is 13:58.06
The men's road record for the distance stands at 12:49, with the women's at 13:54.
The top 25% of runners in age categories from 18 to 75+ will represent their nation at the regionals, before the 10 best men and women in each category advance to the world finals in Italy next March.
The $100,000 prize pool is split between the men's and women's overall podiums, across seven age-group categories. The exact distribution has not been published.
There are currently 20 countries taking part and the list of elite runners registered has not yet been disclosed.
Organisers hope to entice more participants with wildcard entries to events like the 2027 World Road Running Championships in China.
Despite the treadmill sometimes being referred to anecdotally as the 'dreadmill' due to the repetitive and monotonous nature of indoor running, World Athletics president Lord Coe told BBC Sport it's a "great opportunity" to introduce a new audience to running.
"It's across a whole range of age groups. It's very important that we continue to innovate and it's really important that when we innovate, we use technology that attracts more young people into our sport," he says.
The ambition for treadmill running doesn't stop there, with hopes the discipline can also be included in Esport competitions in the next five years.
"We know there are a lot of long-distance challenges but nothing at that 5km mark which is more accessible and that's the real big starting point," Luke Townsend, education manager at Technogym UK, the co-organisers of the competition, tells BBC Sport.
Alongside professionals like Purdue on the treadmills in Southampton were members of the 'She Strides' run club.
They usually opt to run together outdoors but wanted to take part in Run X to highlight how important treadmill running is from a safety point of view.
"Unfortunately, so many women in the winter have to change their routines of running outside because they don't feel safe," says Hollie Thackway, founder of She Strides.
With the clocks going back this weekend, many runners will soon be spending more of their training time in the dark.
Thackway added: "By being able to run inside on a treadmill, it means you don't have to stop your training, you can still be on a programme, achieve your goals and not stop running altogether through the winter, so I think this event is great to highlight that."