You live in a great location for outdoor events (including during the winter :-)). Do you have a date and a course/location for the 5k? If not I might be able to give you some ideas. Once you have the location secured then you will need to get the appropriate permits from your local municipalities and the DNR (Dep. of Natural Resources). You will also need to contact your local police department to oversee the security for the event and your local fire department for any emergencies during the race. Once you have secured all of the above, the next step would be to find sponsors who would be willing to participate and help with your event. If you are doing this event for as part of a non-profit organization or cause you will need to have them aboard before you start any of the above process.
Hiring an event manager or a race event company would be an excellent idea because they can oversee the whole process (from start to finish) and make things go smoothly. T-shirts and other prizes (with the sponsors names and logos on them) should be included in the package. Also, you should have in place all of the logistics that go with such an event (food, lodging, parking, etc.). There are many more things that would need to be considered when organizing a race event, but the above are the most essential to keep in mind.
I hope this helps.