Wow guys it's not that difficult.
So your riding 10 miles comfortable. You need a schedule so you can train and stay on track, so here is a schedule, there are others but this works just as well, see:
Training for a Century Ride
Now note on the schedule that first off Saturdays are your long rides, also note that the first week Sat is 30 miles...I know, that's too much so you reduce all the mileage figures for all the days of the week for that week by 2/3rds so that on Saturday your riding 10 miles; then the next week your riding 20; then the third week your doing 30 and using the schedule for that week just as it's laid out. Thus by the 7th week your Saturday ride will be the 50 mile charity ride and you'll be ready!
Please pay close attention, no matter how you feel stay exactly with the schedule, it is designed to increase your mileage with out making you too tired for the long Saturday rides and without injuries. Don't say one day where your schedule to do 10 miles and you feel like doing 15, stay on the track you will be far better off for the charity ride if you do.
Good luck, you can do this, it isn't a big deal, just follow the plan to the letter and you'll make it.