I totally understand, it can definitely be hard to stay motivated after a long day of work, however all you really need is 30 minutes of intense exercise 2 or 3 times a week, which is only 1 or 1.5 hours a week. The good part is, that once you start going at least 2 times a week regularly, you'll have much more energy and the 12 hours at the office will feel like they just flew by.

However, at the beginning just really focus on why you want to go the gym. What's your goal? The real reason why? Visualize how you'll feel once you achieve your goal? If you were in great shape in the past, think about how great you felt back in those days? Do you feel as good today? If not, is it affecting your confidence or the way you interact with people today? How will you feel getting into the best shape of your life now? And so on. If you can always refer back to these questions, you should be able to find the motivation.

I hope this was helpful, and feel free to ask me any questions you may have and I'll try my best to get back to you :)