For a start, you can watch the course. It's different in every marathon and typically takes the runners from the first lap inside the stadium to a running tour of the host city and its environs. Every marathon requires different kinds of preparation because the character of the courses can vary so much - some with lots of hills, others mostly flat, some with many turns, and so on. Every course will play to different strengths and weaknesses of the runners.

Pace judgement is important in every race longer than 200m, but it's almost everything in a marathon. Runners constantly have to keep tabs on their pace throughout the race to make sure they're keeping some energy in reserve for a strong finish, but at the same time, they must remain alert to the way the race is taking shape and be able to make quick adjustments - or have confidence that they can catch up if a group or individual makes a move. They need to be able to climb hills assertively and descend them without coasting or breaking their pace or stride pattern.

Marathons are a mix of prudence and risk-taking. If a pack of runners goes out ahead, you can actually lose sight of them. The contenders like to keep relatively close to the front and keep an eye on each other, but the packs of runners that form in the first few kilometres generally string out as the race progresses, with the real medal contenders gradually -- or sometimes suddenly, if they're making a gutsy move or simply have run the perfect race -- pulling away from their opponents.

"The last few miles are very tough," says Canada's top marathoner, Bruce Deacon. "You begin to feel cramps and twinges, and your quadriceps endure a great amount of pain. All in all, it is really uncomfortable. Your whole body is screaming at you to stop, and yet you know you must keep going."