A lot of products fail because the entrepreneur built a product that noone wants.
You'd be better off validating your idea before you spend any time building.
Analyse any existing products in your space and contact your target market and ask them if they want your product and if not, why not. You'll learn so much more doing this than you will spending 90% of your time building. :-)
I'd also add that building a pre-launch presence is essential. The more people you can get to sign up to your email list/social media accounts, the bigger audience you will have to launch to.