Since I know which direction you are coming from

I'll suggest that you are better off finding a ride for you and your boat from St. Charles to KC prior to the race. Makes more sense for someone who lives east of the finish line like you do. All you need is a west-bound paddler with extra rack space and room for you.
It's a 4+ hour drive from St. Charles to KC so if you leave your vehicle in KC and have to go back to get it, you are adding 8-1/2 hours of driving to your trip home.