Long distance ride essentials?

Hi all I'm planing to do a long distance ride in a couple of months not an endurance but similar. We will be doing 100km a day, 3 horses a day for 4 days...

I'm allowed to pack 5kgs of gear & that's it....what essential things should I take?

What energy snacks as anything I want to eat during the day I must pack myself?

Any advice appreciated.

HUH ? Didnt understand that "We and 3 horses part" going to have to break down what you are actually doing better.

A group of us will be riding 100km a day, but we get three horses each to cover that distance....so it's not exhausting for horse but will be tough on rider

Medicated powder, sheepskin saddle cover, extra tights, socks, stuffable raingear, some anti chaffing cream, not sure how you are handling the tack situation.