Focus on the competition.
Is there room for another forum of the genre you're thinking? Can you tweak the idea you had a bit to make it more unique?
Perhaps run the idea past a few people, get their ideas on how you can make your forum more unique, and then run these ideas past a few different people to see if they agree.
Then set yourself a target of where you see your forum in 6 months, 12 months, 6 years or something - Do you want huge stats of members and visits per month or would you be happy with a rather small tight-knit community that visit regularly? If you manage your ambitions and expectations you'll manage how you spend your time at the start of the forum much better.
I'll use promotion forums as an example, I have no idea if it's the case now but it was from 2007-2010, people see a forum the size of FP and think 'I want that' so make one. If there's a site the size of FP for promoting forums, what benefits do people get from joining one much much smaller in that genre of forum? Absolutely nothing.
Make sure it's something you're passionate about too, because this passion will help you keep up the hard work when you're distracted by other things in life.
