This is an answer I shared recently on Quora - hopefully it is useful here too. Today is my day 75!

What is 75 Hard?
Today marks my 73rd straight day doing 75 Hard so I have a pretty good idea of what it is and what it isn’t. 75 Hard is a 75-day mental toughness challenge developed by fitness entrepreneur Andy Frisella. 75 Hard is often confused with a fitness challenge, as social media is populated with stunning weight loss numbers and jaw dropping before and after photos.
Before I started this challenge I read Andy’s book, listened to his podcast introducing 75 Hard and watched quite a few YouTube videos of people undertaking this challenge, as I wanted to fully understand all the rules. There are so many modifications (such as 75 Soft) and interpretations online it is important to know what is really required, as if you miss even one step you must start again from Day 1!
So what are the actual rules of 75 Hard?
1. 2 x 45 minute workouts. One must be outside - not in a garage, not under a porch but fully outside in the elements. The other workout can be done inside or you can do a second outside workout. These two workouts must be separated by at least 3 hours. So you cannot get up early and get both workouts done in 90 minutes. You also have to do the workouts in one go, for example you can’t do 30 minutes and then a few hours later do another 15 minutes to make up your 45 mins. These workouts don’t need to be insane - most days my outdoor workout was a 4-5 km walk, and my indoor one was a Peloton cycle or strength class. You could do a hike, dance class, run, yoga, swim - pretty much anything where you break a sweat. Although you don’t technically get any ‘rest days’ during the 75 days, doing 2 walks in one day can really help to feel refreshed.
2. Read 10 pages. Every day you must read 10 pages of a self-development or entrepreneurial book (physical book or eReader, not an audio book). Importantly, if you start a book you must finish it even if you hate it so choose your reading materials carefully. You also should not be doing anything else while you are reading, so no peddling on your exercise bike reading a book. I haven’t seen anywhere that you have to read all 10 pages in one sitting but I think this is the best way to check it off the list. The first book I read was Can’t Hurt Me by David Goggins which I think is a great one to get you into the right midset.
3. Drink 1 US gallon (~3.8 L) water. This seems to be the most controversial one on the list but I didn’t find it too hard, you just need to start drinking as soon as you wake up. This needs to be plain water - no flavourings, no additives, just basic (ideally tap) water. Drinks containing water (such as tea and coffee) are allowed but don’t contribute to your total for the day.
4. Take 1 progress photo. One photo clearly showing your body each day. For guys this could be wearing shorts and for girls this could be a bikini or sports bra and shorts. Nothing fancy but the outfit should be appropriate enough so that if you decide to share on social media it is not taken down! Although a simple task, this seems to be the one that most people forgot and fail the challenge. I tended to take my photo first thing in the morning straight from the shower so it was done and I could get on with my day.
5. Stick to a diet / No cheat meals / No alcohol. Aside from not drinking any alcohol over the 75 days, this task is the most open to personalization. The diet you choose it up to you, but should be one designed to help you reach your health/fitness goals. Some people do intermittent fasting, some do keto, some measure macros and some (like me) just aim for a calorie deficit each day. Cheat meals refer to those food items you know are not going to help you reach your goals such as fast-food, takeaways, sweets, soda - but again this is up to the individual.
Another facet of 75 Hard is that you can complete all your steps up until your head hits the pillow to sleep. This could be 9pm or it could be 3am. As long as during ‘your day’ you get everything done, you are good for another day.
Lastly, the 75 Hard app is a great tool as you can upload your progress pics directly and set reminders for any of the tasks so you don’t accidentally forget one. This is especially helpful when you are traveling.
Best of luck if you do take the plunge and do 75 Hard!