Races as fund-raisers
The purpose of a fund-raising event is to raise funds and those most likely to contribute are existing supporters. The majority have given before and will probably give again...and again, and again.
Private events, like luncheons, dinners, and silent auctions, appeal to supporters. Most people who attend are supporters and their guests. The draw is the cause.
Sometimes, though, organizations produce public events, like carnivals, office parties, and running events. These are meant to raise funds from people who are not supporters. Attendees are drawn to the venue, not the cause.
The organizations raise funds by selling admission, food, beverage, or trinkets (that have nothing to do with the cause) or by amassing enough attendees to make the event attractive to those who do support the cause (like walkers and non-participant donors).
Running events usually are unrelated to the cause or to the organization's mission. Running is no more a charitable activity than is standing at a bus stop or waiting in line for ice cream.
But the venue garners a lot of publicity (attracting supporters and media) and offers financial opportunities for non-supporters (sponsors).
Trying to convert runners to supporters is usually a waste of time. Amassing enough runners to attract supporters and sponsors, though, is a good strategy.