I'm 58 now but when I was 54 I was in almost the same boat with a group ride. At the 42 mile mark you had to decide between a 64 mile or 102 mile finish. I went for the 64 and don't regret it for a minute. At the time it was the longest I had went in a single ride despite doing around 100-130 miles a week. I still have not done a full century although I am considering doing the Seattle to Portland next year if they actually have it. I also live right at the half way point so I'd be able to sleep in my own bed and get a good meal before day 2. We will see how the training goes....