Good for you!! A century is fun, and I think you'll be able to pull it off =)

Several +50km rides behind me at the moment

What's your current max distance, and was it very difficult, moderately difficult, or easy?

How much food should I pack?

Without knowing much about you or your riding style, take the amount of food you'd normally ride with for your 50km rides, and double it + add a little extra.

I've never been good with packing food or water, TBH. Sometimes, I'll come back from a century with most of my water and snacks untouched. LOL Sometimes, I'll go on short rides and will run out of everything.

My habit recently is to keep an eye on my calorie consumption on my bike computer, and make sure that I'm eating MORE than what the display shows. I'll also make sure to carb load the night before and the morning of my ride.

If I do it right, 100km is relatively easy. If I get my calories wrong, it's a very hard ride home.

Do you guys use sports drinks etc for hydration?

No, but I've been considering it just because it gives you calories, fluids, and electrolytes in one go. I'm a cheap bastard, so that's what's holding me back from using them.

I will bring water only, but will have electrolyte sachets in my trunk bag just in case I need it. I also like to bring water only because water only can be used for more than just drinking (handwashing, wound cleansing, etc.).

Any surprises I shouls know comparing the ride to shorter ones?

Provided that you're comfortable on your bike, it's just a longer version of your short ride :)

But... if you'll be out in the sun and heat much longer than you're used to, use sunscreen and hydrate more than normal. I find that the sun kicks my ass, while cloudy rides are much easier. For that reason, I've also been using thin arm sleeves to help with UV and to keep a little cooler.

Good luck!!