If you're doing 70 or even 60 hours per week there are people who are carrying your weight; doing the work you won't. Who are they?
40-hour week people. They're more productive than 70-hour week workers.
Studies have shown repeatedly that people can't work themselves into the ground and do better than, or even the same as, people who don't (glad we have studies for this). There's an optimum productivity of between 35 to 40 hours per week.
More than 40 hours and you start doing less per additional hour up until the point of around 60 hours. After 60, you start making so many mistakes that you achieve less than someone working 40 hours.
Let that sink in: Less. Overall.
Here's the Research
This research found that 60-hour weeks were more productive for the initial 3-4 weeks. But after that, people started to produce less than 40-hour week workers.
There's also this Economist article showing research from Standford University, and this Wall Street Journal piece that shows how many of these extra hours are fabricated. And this piece which explains the origin of the 40-hour work week: "The most essential thing to know about the 40-hour work-week is that, while it was the unions that pushed it, business leaders ultimately went along with it because their own data convinced them this was a solid, hard-nosed business decision."
Oh, and then there's the health effects of longer hours (Forbes Magazine), leading to more sick days and poorer performance.
So when someone tells you they've had a hard week of working 60+ hours, tell them to stop wasting everyone's time.