On Thursday Sonia O'Sullivan spent some time chatting with Greta Waitz, the great Norwegian runner who has won the New York marathon nine times. O'Sullivan and Waitz have known each other for quite a while and the younger woman was happy to sit and listen.

"I just wanted to speak to her. She said things I needed to know. She said get warm and stay warm. She said the second half is tougher, so don't get carried away. Central Park is hilly, but not the worst, yet after 20 miles it probably will look worse. Sometimes it's better facing the unknown than fearing what you know."

That's it in a nutshell. For O'Sullivan this week has been about familiarising herself with the marathon course without building it up in her head as a monster. It has been about learning. When to drink, how to drink. How much drink to submit to the organisers by today's deadline of three o'clock. Simple things.

And today she sits and studies her rivals. She has scheduled this afternoon to sit with her coach, Alan Storey, and partner Nic Bideau and work out the tactics. Two names in particular occupy her thoughts, Lornah Kiplaget and Margaret Okayo. Both, inevitably from Kenya, offer completely different challenges.

Kiplaget will go hard early. The first two miles of the race from the wind-battered Verrazzano Narrows Bridge are tough and uphill and any break is unlikely, but there is good running for miles after that and Kiplaget's style in races is to go. O'Sullivan's decision will be whether to stay with the pack or go with Kiplaget.

The other half of the Kenyan threat comes from Okayo, who runs a different sort of race tactically. Okayo will hang back till the last 10 miles and then begin putting in the work. On the course in New York the last 10 miles are also the most difficult in terms of terrain and wind. Opting to wait on Okayo, who is the defending champion, could be a mistake.

What impresses most of her rivals about Okayo is the fact that in winning in two hours, 24 minutes, 21 seconds last year, she submitted a negative split - that is, she ran the tougher second half of the race in a faster time than she ran the first half. Okayo also won the Boston marathon earlier this year by taking a furious sprint to the finish line.

For O'Sullivan the main challenge lies in measuring her own effort to come in with enough strength to cope with the final stages. That may require going with Kiplaget and hoping to lose Okayo.

In training, O'Sullivan has been running 71 minutes for the half-marathon, and feels that even if the first half of the race slips to 72-minute pace she is capable of maintaining that pace over the second half.

The key points will be the steep, half-mile climb towards the Queensboro Bridge with about 10 miles to go, and then a long, uphill stretch through Upper Manhattan and into the South Bronx, which ends in a steepish climb to the Wills Avenue Bridge which brings the runners into the South Bronx and the 20-mile mark.

A mile or so later, the race is back in Manhattan and loops around down into midtown and Central Park for a finish which is deceptively tough.

The advantages for O'Sullivan are in this year's format which means that the women's elite runners set off in a group on their own at the head of the race.

The leaders will be able to track each other easily, and O'Sullivan has noted that the absence of male runners around her will make it easier to keep a steady pace rather than going with a stronger runner for a while and tiring suddenly later on.

It's a matter of tactics, but also a matter of discipline. Off no training, O'Sullivan ran 2:35 in the Dublin marathon when she entered it on a whim two years ago. She reckons that the event-specific training she has done will get her a time between 2:20 and 2:25 tomorrow. Concentrating on achieving that will probably be the best tactic of all.

In the men's race, Mark Carroll hopes to establish a new phase of his career and perhaps retrieve something from a season which has been disappointing and injury-plagued. The biggest threat will be Tesfaye Jifar of Ethiopia, who appeared to cruise as he ran to a course record of 2:07.43 last year.

Carroll will be left hoping for errors, unless he can produce some astonishing form.