"I'll be more than a little nervous then for sure. There are two times in life I get nervous. One is when I stand on the first tee at a really great golf course, and the other when one of our horses races.
"It is different than cricket because when you own a racehorse you have no control over their performance or the outcome.
"You just hope they get some luck and have a fair crack. And to be honest, more than that, I always hope they are all right after the race.
"Once the adrenalin calms down, the first thing I want to know is how the horse came through it all.
"I suppose that is why racing appeals to so many people, even like the Sheiks, because no matter who you are you can't control the outcome." But racing does share one common thread with McCullum's cricket - the desire to win.
"It appeals to me is because the other side of the coin is a bloke like me can take on horses owned by the Sheiks or whoever and have a chance of beating them. That is pretty cool."
McCullum and Sir Colin are part of a syndicate run by Auckland-based Go Racing, who syndicated last season's Karaka Million winner Ruud Awakening.
A winner two starts ago, Prince Mambo is trained by Steven McKee who says he has "a definite each way chance from a good barrier".
McCullum has shares in around 10 horses "mainly small shares because you still get the same buzz" and realises the enormity of even making New Zealand's richest race tomorrow.
"When you think they sell something like 1400 yearlings every year and we have one who made it to the richest race, that is an achievement in itself."
Of course, there is the small matter of the third ODI against India today, but McCullum says he has been loving the challenge of the series and says his interest in horses provides a healthy mental distraction during his downtime.
Tomorrow's twilight race meeting, followed by a huge Ellerslie party, kicks off the most important financial week in New Zealand racing, with the premier sessions of the Karaka yearling sales starting on Monday.
