You should be building your software. No reason to wait on the answer from YC; if it was good enough to try to get them to invest in it, it should be good enough for you to keep building with or without them.
Hope for the best, plan for the worst, even if you don't get funding you might find yourself with a winner, and your chances of finding funding go up with every hour you put in to your product.
Nothing better than being able to show your product in use by actual customers and a nice start of a growth curve.
It also gives you some real world numbers that you can plug in to your business plan.
This past weekend I started on the prototype again. If not accepted in S10, I hope to re-apply for W10 with a deployed beta as my 'prototype'.