Yeah I was thinking about this last night. I just did a 70 mile ride with roughly the same amount of climbing. Over 100 miles, it's really not bad.

Although "flat" and "hilly" are very much in the eye of the beholder, and very much related to the terrain you're used to riding. The more you're able to ride hills beforehand, the better prepared you'll be.

FWIW I've done the Sea Gull Century which is on the eastern shore of Maryland, where it really is very flat. And I hated it. It was boring and there was no opportunity to coast at all. With hills at least you get to enjoy a downhill now and then.