For me, the first few days didn't feel harder than any of the other times so far really, but I think I was one of the few (extremely lucky!) ones who just sort of slipped into it easily (or perhaps I was so mentally psych'ed up for it I didn't notice feeling bad?)
I have heard from many others that the first 3 days get progressivly harder. After all, I'm sure that all of us at some times in our lives have gone a single day without hardly eating anything at all, and the body has ways of just coping with this; it's when you don't eat anything again in the second day that it's going to start saying "whoa, hang on here, you'd better give me something or this isn't going to end well!" and by the third day it is going to be feeling really desperate - it's only at the breaking point that Keto kicks in as a defense machanism against starvation (thanks to the vitamins and minerals in the packs your body is 'tricked' into Ketosis instead of Starvation mode)
The key is breaking through the first few days, as after the 3rd day you will be into Keto and your body will stop 'asking' you to feed it, as you wil be (technically) getting all the calories you need, its just that you have fooled your body into taking those calories from it's fat stores instead of something you have eaten.
Sadly, the body is merely a pre-programmed machine that will always take it's energy from the easiest sources - only by depriving it of food, and using up the glycogen stores can we force it to start burning it's fat for energy.
So, long-winded answer condensed; be prepared for tomorrow and the day after to be harder than today. But, stay positive, and be even more preapred for the amazing feeling you will get from day 4 onwards, not to mention your first WI and seeing how much you've lost already