This is not the first post in which I will say that starting a new venture will take a lot of time, nor will it be the last. There are lots of good intentioned people who start businesses, especially Internet startups, and let them waste into nothing because they simply don't have the time during the day to give it the proper amount of attention. Life gets in the way.
This brings me to the main point, which is if you have a day job and you need to spend 40 hours a week earning a paycheck, it will be extremely difficult for you to run a business successfully. I don't know about you, but I'm very crabby if I do not get my eight hours of sleep every night. On a weekday this leaves exactly 8 hours to take care of my son, drive to work and back, speak to my significant other about her day, eat breakfast and dinner, and maybe watch a little TV if I can. This doesn't leave a whole lot of time for doing all the things that a business owner would need to do.
I've read a few business books about being an entrepreneur. A lot of them address this problem with a solution that you should quit your job if you want to start a business. I don't know about you, but not all of us have that sort of liquidity or flexibility to do so.
So what's my solution? It's almost exactly like blogging, actually. Just do a little bit everyday. Give it 15 minutes during the days you have to be at work, and an hour on your day off. Keeping the momentum up will allow you to make small progress until you feel comfortable or getting the sort of revenue that you need to go out on your own...