I've only done one as well - the Dartmoor Classic. Either 100km or 100 miles. I did the 100km back in the summer (65 miles). Took me 7 hrs 20 in all, only 6hrs 5 of that was cycling - i had a few stops along the way!!
How fit do you have to be? Well that depends on the distance and the gradients and how fast you want to do it. People say you shouldn't do the full distance before hand, but work up to about 85-95% of the distance, and if you can do that, you'll be ok. I'd only done 40 miles prior to Dartmoor, and if i do it next year, I'll make sure i can do more than that easily, AND cope with nightmare hills!
The Dartmoor one worked that you register the day before (they prefer you to do that, though you CAN register on the day), and you get a timing chip to stick to your helmet and have a number you have to display on the front of your bike. The two distances had different coloured numbers so that marshalls could divert people from the longer route to the shorter one if they weren't going to make the time check. We were given a route card, and though it's pretty well signposted and has marshalls stopping traffic on some junctions (most sportives are not closed road events), it's your responsibility not to get lost. There were just over 2000 doing the event though, so it's pretty hard to get lost!
There were two timing checks for those on the longer route and one for those of us on the shorter route. If the people on the longer route didn't get to the first check in a decent time, they were diverted on to the shorter route. The time check was also the feed station where we could get water / squash / cake / bananas / quiche etc, go to the loo, have a rest.....
You have to carry your own food though and not rely entirely on what's provided cos the food station comes after 35 miles, and also take spare tubes, puncture kit, multi tool, and, cos it was dartmoor, they recommended we take at least a rear light in case the weather got crappy.
From what i can see, most sportives work in a similar way - Dartmoor started rather early in the morning, so if you enter a similar one, get in a few early morning rides to get used to getting up early and putting in effort!
As others have said, and as I'd say, it depends on distance, gradient, and how fast you want to complete the event.