A lot of the advice I've heard is that you need to believe 100% in yourself and your product and commit yourself totally to it. That if you have any fear or doubt then you shouldn't even bother.
I would argue that doubts and fears can be quite healthy, and in fact often an important component of success. Doubts and fears force us to question our decisions and assumptions. If you have no doubts or fears then you are perhaps not as careful as you might want to be with your resources, your relationships with customers, vendors, with your product development, with your hiring, etc.
Of course, you don't want to be so fearful that your paralyzed into inaction, but a healthy dose of concern I believe is critical. I've met a lot of "supremely confident" entrepreneurs, and, to tell the truth I've been one more than once personally.
Though that confidence can give one tons of energy and excitement and be very motivating, I don't find I always make the best decisions when I'm in the throes of that over-confidence. Sometimes that blind confidence needs tempering, and there are few things more tempering than a failure or two.
Anyway, that's my hard-won 2 cents...
Your fear that after one year's effort you will have "nothing to show for it" seems unfounded to me. In my experience having done many of these, the learning curve has always been steepest in the startup environment. While I've learned plenty in consulting gigs, or as an employee, it was nothing like drinking from the firehose that is a startup.
Also, having "failed" at a startup is not the detriment on your CV that you think it might be. To me it signals someone who is motivated, hard working, can juggle multiple projects and has a can-do attitude. It should be a strong asset to your CV. As an employer, I might be wondering if I could keep you interested and engaged enough to keep you long term. So, that's something you'd have to discuss, but that's a good discussion to have.
So, if you're in a position to float your enterprise for the year (you mentioned) have something that you're close to launching and believe in it, I don't see any downside (from an employment perspective) and certainly some upside from that perspective to going for it.