For Keitany, the second fastest female marathoner of all time, 'doing well' could only mean victory, but the 33-year-old will need to overcome some formidable opposition if she is to claim her second title in New York.
Chief among them is Caroline Rotich, the reigning Boston Marathon champion, and Tigist Tufa, who won the London Marathon earlier this year. Others, such as Aselefech Mergia - who has run 2:19:31 - and Priscah Jeptoo are also capable of springing an upset if returning to their best.
Undoubtedly, though, the race is at the mercy of Keitany, who warmed up for New York by routing the field at the Great North Run in September, her winning time of 67:32 leaving her over three minutes clear of the closest challenger.
As she well knows, though, the full 26.2 miles is an entirely different proposition to the half marathon. On her first two attempts at the marathon in New York - in 2010 and 2011 - Keitany could only manage third after failing to measure her effort correctly.
Last year, though, she got it just right, and Keitany is hoping for more of the same on Sunday.
"The half marathon is very different, and the training is very different than for a marathon," she said. "You have to do a lot of long runs. Sometimes I go 30 kilometers, sometimes longer, but I don't run 42 until the race."
Keitany is coached by Gabriele Nicola, and trains alongside a host of other top marathoners in Kenya such as this year's Berlin Marathon champion Gladys Cherono.
On Sunday, expect to say Keitany conserve her energy over the first half and save herself for the closing miles into Central Park; it's the strategy that worked so effectively last year, and she's hoping it will be good enough for victory once more.
"I don't know what the pace will be," she says. "Last year I ran fast at the end and I have to be very careful again because [on this course] it's not possible to run fast throughout."
When the going gets tough on Sunday - as it inevitably will - Keitany plans to draw strength from her two children, Jared and Samantha.
She gave birth to Jared in June 2008 and Samantha in April 2013, which kept her out for all of 2013. "It was tough to come back, but as time went on I got back up to full training," says Keitany, who returned to high-level action last year at the Great North Run, winning in a blistering 65:39.
"It really was tough at the beginning after having a baby, but being a mother is also good. When I'm running I remember them and I realise I'm not running for myself, but also for my family."
Though she has one win from three attempts at the marathon in New York, the city holds a special place in her heart. "It's a wonderful city. I like the fans, and last year I remember coming into Central Park and they were all saying 'go Mary, go Mary'," she says. "I love the New York City Marathon."
And if all goes to plan on Sunday, she may well love it even more.
by Cathal Dennehy for RunBlogRun