Think small
Marathons and mega events get a lot of attention in the running world. They attract thousands of participants, offer lots of prize money, and field a slew of elite athletes who may generate publicity.
But those events are not typical.
The average running event in the United States draws fewer than 250 runners. Most small races will always be small, even if they grow a little bit each year.
You may be better off planning for a field of 250 runners, not 2,500, and not 25,000. Model yourself after events with your level of finances, marketing reach, and personnel resources.
Expect growth to be incremental. Set realistic goals for registration, sponsorship, community involvement, publicity, and everything else. Don't "shoot for the stars" unless that is a practical strategy (it usually isn't).
It's not defeatist, cautious, or cowardly to think small. It's realistic.
You're more likely to be successful if you aim for what's realistic.