When starting my business, I was working as a derivatives trader. The job was stimulating but I did not love it. It was not creative enough. Creative in the purest sense of the world. We did not create businesses, rather create profits through buying and selling.
I had to be up before 6am anyway to get in before the markets opened. We could leave shortly after the markets closed though so I did most of my work time in the evenings from 6pm to 10pm, sometimes later. And then on the weekends (probably another 12 to 20 hours).
I think the important points are:
1. Enjoy the work. If you do then you don't feel like it is work, rather a hobby.
2. Make the most of your 'day job' time. I would squeeze in gym whenever things were quiet. I would answer emails on the toilet. I would read the Financial Times and then sneak in Tech Crunch (I was trading TMT so I argued it was important to view trends).
3. Stay disciplined. A lot of the time I would get excited and stay up later. Rolling out of bed at 5:30am to get into work after being up since 1am does not feel great. You can do it once during a week, but twice and you really do become a zombie for the rest of the week until you get the time back.
I did this for two years until I managed to get some funding to take it full time. It was super tough, especially for my girlfriend. But I loved it because I believed we were building a project that would change the world. Whenever I felt down I just watched SJ's Standford speech and it would pick me up.
"Your work is going to fill a large part of your life, and the only way to be truly satisfied is to do what you believe is great work. And the only way to do great work is to love what you do. If you haven't found it yet, keep looking. Don't settle. As with all matters of the heart, you'll know when you find it. And, like any great relationship, it just gets better and better as the years roll on. So keep looking until you find it. Don't settle."