When I was 28, I had a specific dream (starting a business) but was afraid of the risk. In your case, its not clear to me if you have a dream but don't know how to get there, or if you wish you had a big dream, but aren't sure what it would be. My advice applies more to the former than the latter.
My advice would be:
1. If you are really dissatisfied with your job, look for a way to shake things up. I left my good job to go to business school, then quit after a few months when I realized I'd learn more by trying to start a business. Business school wasn't the answer, but I wouldn't have quit the job directly, so it facilitated things. Its hard to know in advance what will work, but if you are stuck, try something new.
2. Keep your eyes open for opportunities. The business I started was related to experience and contacts from my previous employer.
3. As soon as possible is the best time to take risks and make changes. It won't be any easier to take risks as you add in more responsibilities (house, kids, etc).
4. Do your best at whatever you are doing, but don't worry too much if your plans don't work out exactly as you thought. I thought I wanted to be an industry titan. That was not in the cards. However, once I had kids, it turned out what I wanted was a really flexible schedule. In retrospect, if I was an industry titan, I suspect I would be looking for the exit (actually, given my skill set, I probably would have been shown the exit).
I obviously don't know you, or much about you. But in my case, shaking things up to pursue my dream really worked out well, FWIW.