If you compare the route maps, they're the same route for about the first half of the 34-miler. So you can ride out to that point and then decide which way to go. On these charity rides, they normally ask which route you're doing, but nobody cares if you switch, either. You might want to check just to make sure you don't mess up their "race" results. I suppose registering for the shorter route and riding the longer would avoid that issue.

The speed and distance that you're riding now are about what I was riding when I did my first 100k ride, and I would guess that you can do the 100k okay. If the route turns out to be hillier than what you're used to or expecting, or if it's super hot that day, you might want to reconsider.

One thing to watch is how long the course is open. I note under the FAQ, they say that courses are open until 3:00 PM, which is pretty generous. So if you care to take the time to do a 100k, you're allowed it. And I would figure around 5 hours, depending on how long you spend at rest stops. Leg cramping depends on how well you hydrate and stuff, and may or may not be an issue. (Possibly take your own electrolytes or drink mix or whatever.) Butt comfort varies, but if you're still comfortable after 20-25 miles, I wouldn't expect problems.

I also see they have this set up as a semi-race. On the one or two rides I have done that were set up this way, maybe 10% of the people there were actually racing, the rest were just out for a ride, so don't feel obligated to go faster than your preferred pace.