In fact, most of the stories I've heard of companies doing the 20-hour day thing end in failure. I think more people try it that way.
Actually, I should rephrase that---companies that START OUT with 20 hour days more often than not end in failure.
The reason why is simple: there's no reason why you need to work that hard until you've found a "magic model" and are scaling it.
In other words, here is my blueprint for maximizing your odds of starting a successful Internet company:
a) have stable income.
b) start side projects part time, don't work on them for more than a few hours a day.
c) once one of the side projects is generating significant revenue more than income source A, or is growing like a Twitter, then slowly ramp up to where you're working the 10+ hour day thing
d) sell for bazillions
If you skip A and B, you will likely fail.