If you can do 90, then 100 is easy. As above, set off steady, don’t jump on the back of a fast group too soon if you are not “that” kind of rider.
Ignore the sportive “format”. Some faster riders will want to post fast times – let them get off ahead of you. Ignore the charity riders as referred to above, if you can already ride 90 miles, you’ll be ahead of them after the first hill. That should leave you wandering between groups who are used to riding 70-90 mile rides at a sensible pace, so find a group at your pace, and get chatting – met some really interesting folk on sportives and audax events.
Eat and drink regularly, more and sooner than you think BUT make sure you are using energy products you are used to. Grabbing a load of the sponsors gels and powders made a right mess of one event I rode as they gave me chronic wind and indigestion for about 20 miles.
If you prefer real food and drink, just stop at a village shop/pub/cafe and buy some. You are not a pro racer, so you don’t have to act like one!