Oh well what’s the worse that can happen?
Probably a puncture. If you can do 50 miles on a bike you can do a 100, all you need to remember is “food/fuel”. Always take enough with you and never run out, whatever it is, gels, biscuits, bananas, sweets, etc.
On a long distance ride always aim to finish it with at least one item of food left over. I did Croydon to Brighton and back yesterday, 98 miles, I’ve got one energy gel left over from the ride.
Personally I also like to take caffeine pills with me in case I feel I need the sort of boost a cup of coffee gives, but obviously that’s not necessary and some gels/bars contain caffeine anyway.
Poor bike fit over many hours can show up problems which shorter rides don’t, so get that right – fore-aft adjustments to the saddle, change stem, etc.