the general rule of thumb, I was told is that if you can do 75% of the distance over the same terrain but I think for oldsters or old farts or gray lionesses or whatever label you choose I would tend to think that if you can do 90% of the final total distance and do it without being totally devestated, you should be able to muster the distance on the day.
the one and only 120 mile ride I have ever done took 14 hours but by golly I did it. You can too. Good luck, ride long ride strong.
marni
Katy, Texas
Trek Madone 6.5- "Red"
Trek Pilot 5.2- " Bebe"
"easily outrun by a chihuahua."