I've done quite a few centuries, and the worst by far was on a day with high temperatures. It was the worst because the volunteers manning all the support stations were lost. Route 19 IDIOTS! NOT ROUTE 19A! On level ground, I don't think it would be too bad, depending on the level of support you had - people with water refills, etc. It was kind of funny to us - me and the other leader waited at the 50 mile marker (a painted line on the ground, next to which there were supposed to be a bunch of people with water, fruit, etc.) for the rest of the century riders. We mapped out an alternate route back for the last 50 miles, because we didn't want to be out in the middle of no where with no support in case of any medical problems. About 20 miles later, the state police found us, but those were the best 20 miles. People were outside with hoses & we'd stop to get hosed down, we were able to refill our water bottles, etc.
Right now, I'm not sure I'd be in good enough shape to handle 100 in the heat. On a day like today outside, I'd happily attempt it for a good cause. But, there's no way in hell I'd do it if there were any significant climbs - I'm not in good enough shape to handle climbs AND the distance.