My experience is that you're not going to have the problem as clearly defined as you thought in the beginning. If you have a capable team and a flexible strategy you can and must redefine the problem you're trying to solve as you learn more about the domain. I would add that as a form a failure in startup culture: To get so enamored with a perceived problem and your proposed solution as to ignore new evidence that should alter that first impression. Sometimes the new information leads you to incremental or 'attainable' adjustments and you adapt and move forward, sometimes you need to pull the ripcord and go back to the drawing board.