Someday equates to never. Make a reasonable schedule and stick to it, despite inevitable problems. Note the word reasonable. If you push too hard, you could end up with a death march to an inferior product. Plan mechanically, execute organically .
Few plans survive their first contact with reality.
Don't assume the model you start with is the one that will succeed. You have to plan carefully, but constantly check your plan against reality and be ready to modify it ruthlessly if it isn't working. Make execution the acid test . Careful, intelligent planning is important, but for most entrepreneurs that's the easy part. Execution is the hurdle that separates successful from unsuccessful companies.