Short answer--

Know that your lifestyle working 40 hours per week is NOT possible when working 80-100 hours per week. Once you realize this, learn what needs to change, what needs to be eliminated, and more importantly, how to spend those free hours you do have so that you can continue at this pace in a healthy manner. The ideal daily routine for each person will differ, so figure out what you personally need to do to make it work. Focus first on getting good, quality sleep as well as your morning routine, as this sets the tone for a healthy, productive day.

Long answer--

You've already completed the first step-- realizing that there is going to be a large difference with what your 40hr/wk and 100hr/wk life looks like. Some people are convinced they can continue to live the same life, only to come to the realization all to late and usually in a catastrophic fashion.

This is a topic I blog a lot about, and two of my most popular posts cover this topic in great depth-

Essentially, how to prevent burnout with an ideal morning routine-- My Morning Routine For Energy and Productivity: Meditation, Water and Fasted Workouts

Once you are burnt out, what to do to do recover-- Beat Burn-Out: How to mentally reboot and be productive again

Instead of essentially rewriting the posts, I'll cliff note them here--

Things to focus on in order to prevent a burn-out and operate at full-steam for as long as possible:

1. Wake up early. If you talk to anyone who woke up at 5:30am how they feel at 9:30am, more often than not the answer would be "productive". When you're working 80-100hrs/week, being productive is the holy grail. So moral of the story--wake up early! Just make sure you get enough sleep.
   
2. Meditate first thing in the morning. You don't have to be a Buddhist Monk to have meditation be a useful part of your daily routine. I've never had formal training in meditating beyond a few youtube videos that I link to in the blog post above. Simply sit on a pillow on the floor in a comfortable cross-legged position, and think about nothing by focusing on your breath. I set my phone timer for 15 minutes.
   
3. Stay Hydrated. I get a stomach ache when I consume a lot of water with food in my stomach, so I take the opportunity of an empty stomach in the morning to chug some water. Stay hydrated, plain and simple.
   
4. Fasted Workouts. Beyond the numerous health benefits of working out in a fasted state, it's just also a smart idea for someone with a hectic schedule-- it's disruptive to your day by working out in the middle of it. By working out in the morning, you don't have to worry about going to the gym, changing, showering, changing again, etc etc. Your day will flow much easier by getting your workout done in the morning.
   
5. Eat a good breakfast. The definition of a good breakfast might be controversial, but this is what my typical breakfast looks like 6-7 times per week: 3 eggs scrambled with onions and peppers, a breakfast meat (free of nitrates and other preservatives), and an avocado with a little sea salt sprinkled on top.

Working 80-100 hours per week will inevitably lead to a burn out every now and then. The quicker you diagnose yourself and take actions to recover, the better off you will be. To summarize my second blog post on how to recover--

Things to do when you realize you are burned out in order to recover as quickly and as fully as possible--

1. Disconnect, but not completely. For most people, disconnecting completely is stressful in-and-of itself, but when you get back to work, you're going to have so much piled on your plate that you mine as well just throw your recovery period out the window. Take a nice vacation or staycation, relax, but stay on top of things that NEED to get done, and nothing else.
2. Get to sleep earlier. Try waking up at a consistent time, maybe a few hours later than usual if you have that flexibility, but get to sleep SUPER early. I usually pop a 500mg Magnesium pill at 7:30pm, a 3mg melatonin pill at 8:30pm, and am passed out in bed at 9pm when I'm recovering from burn out.
   
3. Cut out the caffeine. You should be taking a break. You don't need stimulant right now. Give your endocrine system a rest. No coffee or tea, just lots of water.
   
4. Hang out with friends and family. Proven stress relief.
   
5. Workout. But don't overdo it. Go on long walks in the morning. Try a few yoga classes. Stay active, preferably outdoors.
   

In the end, figure out what works for you, but these things tend to work for me, so this should be a good starting point!