Most sportsmen and women, no matter how high they progress up the professional ladder, do what they do because of a deep-seated love of their sport.
Marathoners are different. Even those with a masochistic streak reach a point in the race where it just stops being fun. That old maxim about a fine line between love and hate is at work here.
"At 20 miles [32km] you really want to be done. It's not very comfortable at that point," says Kimberley Smith, who tomorrow will line up on The Mall to run the gruelling 42km race.
"It's a really painful race. Not a lot of fun."
London will offer a flat, possibly fast race, although the 11am start and the potential to be running through the hottest part of the day might militate against scorching times.
Expected to be near the front towards the end are Kenyan Mary Keitany and veteran Gete Wami of Ethiopia, but if the race plan falls into place, the diminutive Smith is capable of making a nuisance of herself, especially if the finish time is around 2h 25m.
"I don't want it to be too fast. Some of the African runners can run a really quick pace. Sometimes they die and sometimes they don't. If it is going out at a suicidal pace I'll have to keep a consistent pace myself."
That requires some discipline. It is not easy to watch others disappear into the distance and keep faith in your own plan. If you think the marathon might give you a bit of time to drink in the sights then you're wrong.
"You're just thinking about the race the whole time really. I try to split the race up in my head. I think ahead to the next drinks station. That's every 5km. When you're at 20 miles, though, and you've got 10km to go, it's really daunting."
It's where athletes, no matter how well-trained or hydrated, start to hit the imaginary wall. Like an invisible force-field from some 70s sci-fi show, you can see runners changing characteristics, like they've changed from sneakers to wading boots.
It's where Smith hopes she'll be able to start reeling in her competitors. Touch wood, she's never really hit it.
"I don't know if I've ever hit the wall as bad as some people. You see some people running and they hit it and they can barely move. I'm more likely to have a gradual slowdown."
This is Smith's third Olympics. The 30-year-old Rhode Island-based runner competed at Athens and Beijing on the track and has Oceania records for the 3000, 5000 and 10,000m.
"I am starting to feel more like a road racer now. I really like running on the roads and feel a lot more comfortable. I can't see myself ever going back to the track."