Lots of people are suggesting quitting. Here's a better idea:
Are you really working all of those 12 hours? I imagine not. Probably taking a lunch break, right? Eat at your desk, use your lunch break to work out. 30min a day of strenuous stuff is all you need. Here's an example:
You don't need to go to the gym; you can do it all with bodyweight exercises: pushups, pullups (get a pullup bar that sits in a doorframe without being attached to anything, like Iron Gym, so you can put it up when you're working out and then take it down), Hindu squats (google it) and crunches will get you 90% the benefit of a more thorough workout. If you want some variety, do yoga.
5min to change, 30min of circuit training (google it) with these four exercises, 5 min to change back. If there's a shower onsite, then +10min to shower, and you still have 10min left to relax and start your lunch.
There is never time to do it right, but always time to do it over.