I think Paul Graham summed up the situation fairly well when he wrote: "Economically, you can think of a startup as a way to compress your whole working life into a few years. Instead of working at a low intensity for forty years, you work as hard as you possibly can for four. This pays especially well in technology, where you earn a premium for working fast." and this: "Combine all these multipliers, and I'm claimin…
You have to remember that PG is a startup investor, and has a vested interest in making you feel as though you need to code for 14 hours a day when he's given you a seed-round investment. The overwhelming odds state that your startup will fail, no matter whether you spent six hours or fourteen hours every day working at it.
Working hard is definitely a must, but I think market timing, number of competitors, and other features have more of an impact on success than whether you put in another 4-5 hours a day. In some new markets, there are so few competitors or such a huge distance between the leader and the laggard that hours worked doesn't even matter.
In some cases, capturing mind-share does enough to get you further resources and that alone buys you luxuries that free you from 14 hour days.