I framed houses for eight years dude, and it is not the same kind of mentally draining work that sitting in a law office entails. I got a great job when I graduated, and I'm glad to be here, but you don't sound like you have any reference point whatsoever with regard to the practice of law. I promise you, carrying lumber and playing with Paslodes all day isn't half as exhausting as this. It's just mental fatigue instead of physical fatigue.
IMHO, framing houses is quite easy and is really fun. I looked forward to framing jobs, I even built a government contracted kill-house once. At first, I mostly did demolition, which is the physical part. We are not talking about painting here. Being in the Bay Area, you can't always use a bobcat, especially when working in the Oakland Hills, or when you are working so close to the house. So try digging all day for a week straight. Try crawling under a house with only about 3 feet of height and doing electrical or plumbing in those conditions. You have to use your mind too. My father would always tell me "Work smarter, not harder".
I didn't say it is the same kind of mentally draining work that sitting in a law office entails but on the contrary what if one prefers that? I like challenging work. I like being exhausted from work.
The main point I tried to make is that I dealt with very long hours so it is not something new. The time flies if you are working. If you get frustrated you will stress yourself out. If you care too much about what can go wrong & how you can't fix it, you will enhance the fatigue. It is all a mental game. Also, it is not just physical fatigue. When someone hires you and they tell you what to do, that is 90% physical 10% mental and if you make a large mistake, you might or might not lose your job depending on the circumstances but there isn't really any liability attached.
When the whole job is in your hands and you are responsible for all the employees and their mistakes, you cannot say there is not any mental fatigue attached to that. You have to account for everything the job entails. Most of the time I was in way over my head. This has thought me that I like being under pressure and I do good when pressure is applied, it is what I thrive on.