I usually only ride two days per week and I'm able to do 60 mile/5-6 hour rides. My weekend ride is typically longer, 40 miles or more, while my weekday ride is shorter, about 1-1.5 hours depending on the weather. So I would suggest trying to ride more than once a week, but I think twice a week should be okay if you're not able to ride more that that.
Usually charity rides have rest stops with food and drink -- does their website have any details on the routes and support they provide? If there's no planned rest stop, I would bring my own snacks and stop once or twice to eat something, or find a convenience store along the route (using google maps before the event to find one). Also make sure you have enough to drink for 3.5-4 hours if they're not providing any stops where you can refill water bottles.
You'll want to make sure your bike is in good working order and have the supplies and knowledge to change a flat tire on the road if needed. SAG support at organized rides can sometimes take a while to get to you, so it pays to reduce the chances you'll need to call them for help.
Good luck!
- 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