This race means more to the 30-year-old than any she's ridden in during more than a decade in the saddle and she says it will almost certainly be her last appearance in a major race.
A shoulder injury and more importantly now being committed to a fulltime career as a horse trainer will see Houston virtually retired after today.
"I will get my shoulder operated on after this race and I might occasionally ride one of the horses we train, but this will certainly be my last ride in the Great Northern."
Notice she didn't mention "Steeplechase" after that statement.
The Great Northern Steeplechase is so highly regarded on a scorecard that, even though there is a "Hurdles" equivalent, Great Northern means only the Steeplechase. It's like Bart for Bart Cummings.
This gladiatorial contest with its extreme 6400m distance and three times over the feared Ellerslie Hill is world famous. Former New Zealand racing editor Lex Nichols these days shares his time between Australia, Rarotonga and Las Vegas and loves this one race so much he has made a special trip to his former home to be at Ellerslie today.
Shelley Houston has not won a Great Northern.
She probably should have - another horse, Climbing High, kept her from being in the saddle occupied by James Gillies when Ima Heroine won this big race two years ago.
Ima Heroine's legendary trainer Ann Browne originally talked Houston into being a jockey when she preferred life as an artist and an eventer, but took Houston off Ima Heroine two years ago when she couldn't commit to the mare for every start that campaign because of Climbing High.
"I should have also won one for Mrs Browne on The Storytella. The first year he finished second and the track was seriously good [2005]when he needed a bog. I finished a third and a fourth on him in the two Northerns after that and he didn't get much luck either time."
Ima Heroine is these days around average height on her wither, but overall lacks size.
Luck this time, with 68kg on her back, will be critical, says Houston.
"She's an awkward horse to ride - she's little, but she jumps big.
"Good 'chasers are usually fluent and land over fences galloping, but she's not like that. She lands flat-footed and you have to kick her up to full speed again."
Two weekends ago Houston had planned to stay in Auckland to party on the Saturday night, but after finishing second in the Pakuranga Hunt Cup on Ima Heroine she said her legs were so tired from kicking the little mare along she had to return to Cambridge.
That should have been a winning celebration.
"Going up the Hill the last time she was travelling much better than anything else, but from that point everything went wrong when I tried to get her runs through the pack."
Even after that Houston says the mare would still have won but for being turned sideways by a rival on landing over the last fence. She finished just 1.5 lengths from Snodroptwinkletoes.
But to the horsewoman that means little compared with today's race.
Just how much does it mean?
Well, she turned down the $100,000 Great Northern Hurdles rides on Calgacus, Flavour Flav and Conquistador because she aggravated her shoulder in a fall in a steeplechase at Paeroa on Monday, but also so she couldn't be injured and unable to ride Ima Heroine two races later.
There is something very special about a little horse - particularly a little mare - overcoming what seems an impossible task.
Houston loves her. "She's only a little tot, but she's a brave, brave girl.
"She gives you 100 per cent all the way through the race then she'll find more when you ask her."
Today couldn't be a better scenario to go out on a winning note.
"I've had a hell of a great time riding more than 100 winners, but this is the Holy Grail. It's what I've wanted since I started.
"I'll be complete."