Marty Dent is used to pushing his body to the brink of exhaustion, but the Olympic marathon runner admits he faces the unknown as he prepares for a 100-kilometre slog next month.
Dent is in training to compete in the gruelling event at the Stromlo Running Festival on February 16.
But his hunger for extra kilometres and hard training means he will miss the 12-kilometre world cross country championship selection trial at Mt Stromlo on Sunday.
The 33-year-old made his Olympic debut last year when he raced in the 42-kilometre marathon around London.
But keen for a new test, Dent will up the workload in a way he's never experienced.
''It's going to be a challenge and something different,'' Dent said.
''I'm just trying to have a crack at a pretty fast time … I might crash and burn. The pace to run a really fast time doesn't seem as intimidating as a really fast time in a marathon.
''It mightn't work out, but I'll give it a go and see how I go.''
Dent opted against running in the cross country race this weekend because of his rejigged schedule this year.
Wife Kathie is pregnant with their third child and is due to give birth in April, just after the cross country world championships in Poland.
His absence will open the door for fellow Canberran Brett Robinson to chase a top-three spot.
Two-time Olympian Collis Birmingham headlines the field as he chases selection at his sixth consecutive cross country world championship.
Canberra's Hannah Flannery will be vying for a spot on the national team when she lines up in the open women's 10-kilometre event.
The top three in the men's and women's open events will be selected to compete at the world championships.
Robinson has been training with Birmingham in Ballarat since October, including the past month at Falls Creek for altitude training.
He hopes it will help him clinch a spot in the top three this weekend and earn selection for the athletics world championships in the 5000 metre event.
''I just needed a bit of change to get me to the next level,'' Robinson said.
''I'm training with two of the best runners in Australia [Birmingham and Ryan Gregson], so it's definitely been good for me. It will be tough to win this weekend, but I'll do everything I can.''
World cross country selection trials at Mt Stromlo. First race begins at 7.30am.