I tend to agree. I prefer to do closer to 60 miles in training for a ride that long but 50 should be sufficient. From my experience, terrain is more important -- if the charity ride is hilly, you'll be very glad if you ride lots of hills beforehand. Also when you only have time for a short ride, you'll get more training from hills than from flats.
Also remember that a charity ride is not a race -- just ride your pace and don't worry about anyone else. I used to do the Livestrong Philly 70-mile ride and every year I saw the same group of guys doing the full century riding at my relatively slow pace of ~13.5 mph. They knew they would be among the last riders to cross the finish line but they didn't mind because they didn't worry about anyone who was faster than than they were.
BTW I saw in another thread that you're slow on hills -- I'm the same height as you and am in the "normal" weight range but am also slow on hills. But I'm more interested in having fun and getting some exercise and enjoying the scenery than I am in worrying about faster riders or what anyone else is thinking about me. It can be frustrating to get dropped when you were expecting the group to ride your speed, but when that happens I figure at least I can sing out loud while I ride without anyone complaining. (And yes I have been known to sing out loud on long rides.)
- Gray 2010 carbon WSD road bike, Rivet Independence saddle
- Red hardtail 26" aluminum mountain bike, Bontrager Evoke WSD saddle
- Royal blue 2018 aluminum gravel bike, Rivet Pearl saddle
Gone but not forgotten:
- Silver 2003 aluminum road bike
- Two awesome worn out Juliana saddles