Why take part in a fun run?

There are several reasons. Some people enjoy running, and the marathon is basically the ultimate run that a person can achieve. Some people do it as a challenge to be able to train and complete a full marathon. Others do it for the satisfied feeling that comes afterward and to be able to say they have run a marathon. Some take on marathon running as a way to become healthier people. And unfortunately some are just really competitive and do it to beat others (probably not to get 1st, but to beat someone they know).