Anyone that positions a workout program to you as "easy" is flat out lying to your face. A workout program can be "easy" by way of using basic exercises, or following a simple structure, but no program will ever be physically "easy." And if it is, you're not working hard enough to see results. That, and that alone, is probably the most important concept to understand and accept before even starting any plan, whether that's the 21-Day Shred or something else.
Like any training and diet program, you'll be working out hard, and need to be eating less food than you are now in order to lose weight. Fact: sometimes you will feel hungry. There's no avoiding that. Your body is accustomed to eating a certain amount, and now that amount is going to be reduced. There are, however, ways to fight the cravings to cave.
If you're specifically following The 21-Day Shred, there are quick workouts in the mornings on an empty stomach. Why would we do such a thing? Well, there's been some recent studies that suggest it can help intensify the fat loss process. You can read more about that theory here. You can also take a look at what some of those morning workouts will look like. But, back to your hunger issues. First solution: make sure you're drinking enough water. This will help you with feeling full. And the second solution: slowly sip on a half of a serving of a protein shake. This way you're providing your body the nutrients it's asking for, without really filling up with empty extra calories.
The next challenge some of us face, regardless of how physically conditioned we are, is soreness. We're talking soreness, not pain. Make sure you know the difference. Chances are, if you're new to lifting, or the gym, you're going to be a bit more sore than the next guy. And for the intermediate guy, if you've been doing the same thing for months or even years and change it up, chances are you're going to be feeling things the next day too. While the best solution to (and prevention of) soreness is simply being well-rested and well-hydrated, there are other ways you can facilitate the recovery process, such as yoga and foam rolling. Work in some of these yoga poses and these foam rolling exercises at night or during downtime at home. Another option? Treat yourself with a massage.
Finally, when you're following any kind of aggressive training program like The 21-Day Shred, the accumulation of training volume simply adds up. Day after day you're going to your job for a full day of work, then you're putting in the hours in the gym and on the pavement. It's completely normal to feel tired. Again, this comes down to staying hydrated, but more importantly, actually getting enough sleep. It might be a wise decision to get to bed an hour earlier than usual. And before even starting your program, get yourself into the new sleep pattern a week or two in advance. Have trouble sleeping? You could be all bound up and stressed out. Use these quick relaxation techniques. Or check out some of the other less-obvious ways to fall asleep faster.
While the task at hand may seem daunting, if you can push through, it just gets easier from there. Best of luck on your way to being 21 days leaner and stronger.