Insurance
You must have insurance for your event to protect you against a potential law suit. The waiver of liability that runners sign is not enough. Also, the waiver only applies to participants who have paid an entry fee (or invited runners who don't have to pay). It does not cover spectators or other non-participants, such as occupants of baby strollers.
Insurance policies offered by individual companies are likely to be wildly expensive. If you are a YMCA or a municipal government, your umbrella policy may cover events on public property. General liability policies usually do not cover races.
Contact USA Track & Field (USATF) and apply for a "sanction" (that's different than a course certification). A sanction enables your event to fall under the protection of USATF insurance.
Another option is Road Runners Club of America (RRCA) insurance. You will need to join RRCA and then purchase the insurance. Membership is available to events, as well as to running clubs and other groups. This is a good buy for organizations that produce more than one event during the year. Insurance will cover all of your events.
Don't try to save money by avoiding insurance. The small cost today can protect you against major expenses later.