Does a "challenge" help?
Many runners are looking for a relatively easy race. They don't want a challenging course - running is enough of a challenge. "Flat and fast" is what they prefer.
Others, however, want something more difficult. Running, to them, is a conquest - doing better than before and doing something that ISN'T easy.
The Challenge of the Bluffs Run, in Alton IL, offers something that requires an extra effort. To runners accustomed to the flat terrain of the midwest, any elevation - even a bridge over a highway - is considered a hill.
But there are some genuine challenges, even in the Prairie State. The Mississippi River runs along what locals call "river bottom" (this is the area that floods), with high bluffs lining the river, sometimes several miles away.
A run up the bluffs is taxing and challenging. If you want an experience that appeals to serious runners (even slow ones), find something difficult or unusual (but not nutty). Market it to running clubs and races longer than 5k - even if your event is a 5k.
A lot of runners will appreciate the opportunity to push themselves a little bit more.