You are. The worst possible thing you can do at a startup is plow ahead and put in hours like crazy. Just like Steve Jobs killed product lines, or an engineer removes thousands of lines of code, you can remove 100 hours from the next month if you think about it carefully for one hour.
I'm pretty sure Steve Jobs put in 16 hours a day when Apple was still a startup. I'm not saying you should do that, but it might not be wrong to.