Is it walk or a run?
Some events attract far more walkers than runners.
The Y-ME Empowerment Run, in Chicago IL, typically fields six times as many walkers as runners. At the Revlon Run for Women, in Los Angeles CA, 85% of entrants are walkers (although they're all timed). That percentage is even higher at Race for the Cure.
Cause-related events, especially those dealing with breast cancer, draw huge numbers of supporters who walk the course and contribute to the cause. At most races, though, participants show up to run, not to contribute or raise funds.
When walkers outnumber runners, it may be time to consider dropping the run and hosting only a walk. Few walkers generally participate in races, since the events are designed for running. When they show up in large numbers, it's a sign that your event has tapped into a different (and more populous) market.
Walks require less infrastructure - no timing or results, often no aid stations - and draw from those supporting the cause or the organization hosting the event. A walk may be more profitable than a run.
Just don't call it a run/walk (more about that another time).