I don't know about all charity rides since the length and number of days can be different. I'm guessing the multi-day rides have breaks fairly far apart because they cover so much ground day after day.
Generally, all 1-day organized century rides will have various lengths as you know, half metric, metric, 100m, etc. and the rest stops are about 15-20 miles apart. This varies based on available of parks, schools, or whatever that can provide the facility. I'd say more than 20 is vary rare.
I'm guessing that the 30 mile training rides are at a faster pace than you do when you are riding alone. This is a good thing. Mixing up your pace, doing intervals, etc. is a great way to train. Try upping your mileage (maybe try 40) at your pace not the training group's. I'd bet that if you ride at an easy comfortable pace and take breaks, you could actually do a 50m ride now, you'd just be tired. Adrenaline and determination really come into play on the big day. Pick one that has similar terrain to what you already ride and go for it! We're behind you -- just visualize us collectively pushing you along!