I'm not sure how long your Saturday rides are, but jumping to 75 miles may be a challange unless you pace yourself. The supported events I've attended all have rest stops at 10 to 20 miles. While water and sports drink is usually standard, the food offered ranged from fruit, nutrition bars and peanut butter sandwiches (high) to a box of sugar cookies (low). I'm saying this to say, bring your own power bars, don't count on them for food. But you should be able to count on them to refill your water bottles.
Oh, I almost forgot, bathroom facilities are optional, some have it, some don't.