It has been for me so far!
My 15 year "day job" starting as a developer and moving into technical project management has given me some advantages in terms of being able to structure the work ahead, as well as actually execute the work to be done. However, even with that, starting up my own business has redefined the cliche "there aren't enough hours in the day". The business experience from my past work also helps immensely in terms of articulating my value-prop to different constituencies, the sales process, managing/hiring staff, and marketing & communications in general.
I can honestly say that starting my business asks more of every skill I have... yet that's also the attraction!
So you have an idea... what is the value-prop to your customers; what about your providers? Can you put together a compelling elevator pitch? Can you sell? Are you a developer? Can you make decisions and presciently see their implications further down the road? What about budgets, and financial projections? How well do you know your target industry, customers, and competitors? Do you have staff, know staff, or do you need to find those people? Have you ever managed a project of any kind?
These are the things that you'll face -- no one knows them all and so the idea of "knowing what you don't know" is a challenge unto itself. The point here is that while any smart start-upper is/should be capable of learning on the job and extending their talents into other areas, it all takes time.
Some days I have a plan with three action items on it, but the first item ends up being much deeper than initially thought, and takes the day... tomorrow starts two items behind. This is also where decision-making comes into play. Should you cut your effort on that item or is it wiser to push through? Sometimes I hate that thinking can take longer than I planned for ;-)
For my current start-up I have cleared the way (of a full-time job and other consulting gigs) to make sure I give it a full-court press. It also helps that I have some funding, but I'm still looking for that elusive 35 hour day!
You also know a truth that will be revealed to all part-time start-up founders eventually: there is no progress if you are not making it (I hate that one).
Best of luck!