Many of us, including myself. We start a forum without much thought. And I am glad it is like that, otherwise one would get
action-paralysis.
I believe in progression, things change over time and it takes it's natural course of life.
As mentioned by
@AWS, many times the hard part it's just to pick a topic/theme/niche. But hell, shoot for the moon. Even if you miss, you'll land among the stars.
What I give more weight is, just start....
Starting a forum, takes hard work and long hours/time. It's better to do something you enjoy doing or it will be easy to quit along the way.
But that same question is the one that help you to grow as an individual and eventually reflected in your community.
What was hard once, it becomes natural and easy with enough practice.
So, everything can be hard at the beginning.