I've been pulling 12-16 hour days, nearly 7 days a week for the last 12 years.
You have to love what you do. It has to be what defines you as an individual to pull that off with any kind of success.
It'll kill you though, I have more medical and mental problems than I know what to do with, all stemming from the damage my passion/obsession causes. But that's the cost and I gladly pay it. Why? Because it's what I am, it's what I do - and I'm pretty damn good at it.
How do I do it? Marijuana mostly. I don't drink caffiene (nor alcohol or any other types of drugs/stimulants). Caffeine causes peaks and crashes which make doing 16 hour sprints impossible, so I steer clear of that and I also minimize sugar intake but have noticed a semi-regular pattern of minimal sugar intake during the day - usually in a sweetened beverage or a fruit smoothie or eating fruit - although at 4am I will sneak a snickers.
I also tend to eat a lot of smaller meals throughout the course of the day, which helps keep me energized without making me lethargic.
Also, a few brisk walks during the day are good to clear the head and give me time to isolate my thoughts about what it is I'm working on.
It helps to start the day out with a list of things you want done, with the understanding that you can reshuffle/prioritize that list at any point, so long as you have a list.
But it really does boil down to the marijuana. It does a few things for me, cheif among them is that it sort of clears my brain and lets me really focus on what I'm working on. I'm disciplined enough at this point in my game that I can do this fairly easily. The other thing marijuana does is make me slightly lazy - which actually has the effect of me writing significantly less code to do the same thing, so I gain a LoC metric there. It also forces different perspectives when analyzing a problem so it's fairly often that I find holes in whatever system I'm developing that I might have overlooked otherwise.
Note, I'm not advocating you start smoking weed at work. I'm the boss at my company, so it's kosher, but most places - no way, you'll have to find other ways to achieve the same results.
Listen, if you aren't passionate about what you are doing, don't fine any thrill in it and are worrying about how you're going to pull of an 8 hour day, then maybe this isn't the right career for you?