If I road with a group but not a paceline, but just others around could I expect more than I think. Right now maybe on a good day with lots of rest before the ride I could go 17.5.
I quit trying to ride fast after causing my heart valve to start leaking after I had it replaced in 2016. I had regained the ability to ride 50+ at 20mph - but then I eventually went into A-fib and "five-hour" rides are not on my menu anymore. ( I was 62 at that time)
My impression of your comments leaves me thinking that you want to find a "golden" pace-line or group that will get you through a 100 miles with the least possible strain. (good luck with that) My experience would suggest that you need one or two really strong riders that would concede to your ride abilities and wishes and give you the chance to work or sit up on your behalf as the ride unfolds. Otherwise you may get with a group that will hot-dog flats or ride grades unevenly.
You may be able to get another five hour century but - it is doubtful you will ever see your best result without a strong rider (or riders) thoughtfully pacing you.
.