"If the knee doesn't hold up in the marathon that would be the worst thing ... but if it can get me half way, I can handle the pain from half way."
O'Callaghan said he believed "anything can happen in a marathon". He was confident he could stick with the other top elites tomorrow.
He said his big goal was to record his first win at the marathon after falling just before the finish line in 2009, and placing second.
"It has always been the goal ever since I didn't get there."
The local roofer is Rotorua's best chance of winning the 50th anniversary race but he will be up against a tough group of elite runners.
Three-time winner Phil Costley (Nelson) will be among the favourites and has a chance to match his sister Jillian's record of four titles at the Rotorua Marathon.
"My sister has won it four times before and our family has been pretty highly associated with it, so it is either me or nobody [from the Costley family] for this year's race.
"There's been a Costley on the podium for 12 of the 49 years, so I'll try and add another one for the 50th anniversary race."
Now 44, the veteran athlete has targeted the 50th anniversary race to add to his previous successes in 1997, 2000 and 2012.
Other dangers in the men's race include Murupara farmer Sjors Corporaal, who is an experienced trail runner.
Other contenders include Tauranga's Craig Kirkwood, Japan's Masataka Uchino and two-time New Zealand marathon champion Matt Dravitzki (Wellington).
Last year's champion Sam Wreford (Timaru) will not be racing as he has turned his focus overseas.
Bay of Plenty runner Jessica Ruthe is among the contenders looking to claim the women's title at the 50th Rotorua Marathon.
The Tauranga Ramblers runner won the Auckland Marathon in 2009 and will race alongside a strong women's elite field tomorrow.
Other top contenders include defending champion Erin Furness (Hamilton), Auckland's Lisa Robertson, Dunedin's Shireen Crumpton, Stewart Island-based Klaartje Van Schie and two-time Olympian Liza Hunter-Galvan (Manurewa).
None of the runners, men or women, competing have qualified for the Commonwealth Games in July and it is extremely unlikely any will meet the qualifying time this weekend on the tough Rotorua course.
The Rotorua Marathon will double as the Athletics New Zealand Marathon Championships this year.
The marathon gets under way at 8.20am tomorrow at the Energy Events Centre on Queens Drive.
