MELBOURNE (Reuters) - Jarryd Hayne's bid to play rugby sevens at the Rio Olympics is now literally an uphill battle, with gut-busting runs up towering sand-dunes on the agenda during a six-week boot-camp with the world champion Fiji team.
Used sparingly during the World Sevens tour's London finale at the weekend, the former NFL running back has no further opportunities to impress Fiji coach Ben Ryan with on-field performances.
That leaves Hayne's audacious bid precariously pinned on his work at training, which by itself may prove an insurmountable slog given Fiji's stable of established stars.
"Our cupboard is not just deep, it's absolutely littered with fantastic talent," coach Ryan said in London, where Fiji secured their second straight world title.
"We had a very light week this week ... when we're running up 100-metre sand dunes, when these guys are the fittest rugby players Fiji has ever had and they're still throwing up after sessions, his lactate's going to be going into overdrive.
"Whether he'll be able to tolerate that in a short time frame is a serious question."
Last year, Hayne defied the odds and an army of sceptics to make the 53-man roster of the NFL's San Francisco 49ers, less than a year after quitting Australia's top-flight rugby league competition.
But Hayne allowed himself just a few months to get up to speed with sevens in time for August's Olympics, setting himself a task that is physiologically far-fetched, according to experts.
"Bsic physiology will say he is going to struggle," Donny Camera, a research fellow at the Australian Catholic University's Centre for Exercise and Nutrition, told Reuters by email.
"When considering the divergent extremes of exercise performance between NFL, which is intermittent and involves repeated short sprint efforts, compared to rugby sevens where the game is more endurance based with very few recovery breaks."
Hayne would need at least three to four months of endurance training, leaving aside an extra month for simulated match practice, Camera added.
The failure of Quade Cooper, a highly skilled veteran of 58 rugby union tests, to make the transition for the Australia team earlier this year does not augur well either.
Hayne's sevens debut in London left few in doubt the Sydney-born 28-year-old has the skill and pace required to become a successful sevens back in time.
His defensive prowess was also on display, most notably when he saved a certain try with an ankle-tap tackle on Frenchman Alexandre Gracbling.
In addition to his lack of fitness, though, some observers were less than impressed his work at the breakdown, an area of the game that does not exist in rugby league.
"You just saw how difficult it was for Jarryd out there," George Gregan, one of Australia's finest rugby union scrumhalves, told Sydney's Daily Telegraph newspaper.
"You just can’t come in at short notice and pick up the game easily like that, as wonderful an athlete he is.
"I think it is unlikely he will make the Fiji Olympic squad, but Ben Ryan said that he has six weeks to prove himself.
"One thing is for certain: Jarryd likes a challenge."
Editing by Nick Mulvenney