Depends on your age, conditioning and attitude. When I was in my 20s I bicycle commuted 20 miles a day and rode a bit more on weekends. I had no problems with our club and big event century rides at a moderate pace. A few times a year we did rides of 75, 100, 125 and 250 miles (the latter was a two-day event). I don't recall any particular problems, just being a bit leg weary at the end, but we recover quickly when we're young.
Ride your own pace, don't try to keep up with faster riders even if they invite you to draft. Once they push past your comfort zone it gets dangerous if you're fatigued overheated, dehydrated and riding faster than you can manage with riders you aren't familiar with.
Most large event rides begin with club members, friends, etc., deciding to ride together... which quickly falls apart as their comfort zones differ significantly. But in most large event rides it's possible to form impromptu small groups of riders who match each other's preferred speed/effort level. Some of my local friends encountered that during their first century this weekend at the Hotter 'n' Hell Hundred in Wichita Falls. That was my experience in the 1970s-80s in SoCal. Our group started together, but gradually drifted apart during the ride. I was slightly faster than average, then and now for my age, and usually ended up riding with folks I'd never met before but enjoyed some casual banter to pass the miles. On one ride in Baja I rode alongside Bill Walton while he was on injured reserves with the Clippers. Bill was a bit stronger, surprisingly so on climbs for such a big guy with a tall bike, and eventually pulled away.
But 40 years and a bunch of injuries later, I'm done after 50 miles. Due to neck injuries I can't keep my head up much longer than that, regardless of the bike -- drop bar, hybrid, doesn't matter.
I bailed on the Hotter 'n' Hell Hundred this weekend because I figured it would be a waste of money if I couldn't finish the century. But after a casual group ride Friday evening that got me 35 miles between the group ride and commute there and back, I decided to add another 30 or so miles. It wasn't bad until I hit the 50-55 mile total mark, then it was just painful and unpleasant. I finished with around 66 miles.
Unless I'm willing to switch to a recumbent, or there's some miraculous spinal surgery to fix my busted up neck, I won't be doing any more attempts at rides longer than 50 miles. And considering how much worse driver behavior has gotten here -- dramatically worse since the pandemic and economic crisis -- I'm not willing to ride a low profile bike with my head at wheel height to motor vehicles.
During our casual group ride Friday we had a guest from out of town join us, with her recumbent. One of the other regulars and I hung back as tailgunners, staying beside or behind her to run interference with traffic. Sure enough, we encountered one carful of belligerent idiots who swerved toward us yelling the usual idiot stuff -- git on tha sidewalk/playground, etc. I doubt they even noticed the recumbent rider, despite the flags.
Another bit of common advice is to avoid making changes to your fluid and fuel intake. Just eat and drink whatever you're accustomed to. Most electrolytes, gels, etc., are pretty much the same stuff. But some use sugar alcohols -- especially maltitol -- which can cause digestive upset in some folks. You don't want to find out during a long ride with the next rest station or porta-potty 10 miles away.
I usually stick with Clif bars and gels because I'm familiar with 'em and have no problems. I'm not too picky about electrolyte drinks and mixes, as long as they have a little sugar to aid with digestion. Sugar free electrolytes cause bloating for me. A little sugar aids with osmolarity. The WHO standards for oral rehydration solutions usually recommends sugar equal to salts. That turns out to be pleasantly sweet but not icky sweet like most soda pops and "fruit" drinks.