Running specialty stores
Your marketing efforts should give priority to businesses and organizations that attract runners - those people who are most likely to participate in races. Running specialty stores are among the best venues for reaching runners.
Running specialty stores focus on the needs of active runners, rather than the general public. Some also service swimmers, cyclists, or other sports participants. They are usually very knowledgeable about footwear and other products used by recreational and competitive athletes.
All runners need running shoes. Many recognize they need other things as well - apparel, skin balm, energy bars, race applications - and those items can be obtained at running stores.
Many running stores are extremely savvy about marketing techniques and stage races, training runs, fun runs, clinics, information displays, and other activities to draw people into their stores. Many running stores even fulfill runners' needs for clubhouse gatherings by maintaining volunteer organizations. Some have close working relationships with running clubs.
So, talk to the running stores in your area. They may help market your event and may even have other ideas about how to partner with you. Check all of the running store lists, since they may not be complete. Contact Running Network and Runner's World, or your local running organization. Chicago Area Runners Association, for example, maintains a list of running stores in the Chicago metropolitan area.