Sorry but that's just flat out wrong. There's a few pro's who have cadences down in the 50-60 range and do well enough to be a pro. There's plenty of people here who will grind out a mountain pass at 50-60 rpm for an hour as well. Don't think that 90RPM is some magical number, it's not. It's just the 'typical' range where the average person is most efficient. It's not the aerobic threshold, nor is it a set number for everyone.

To the OP: If you're comfortable spinning at 110rpm then spin at 110rpm. You might want to experiment with some of your 1-2 hour rides and see how you feel and if you're any faster spinning at 90-100rpm if you're worried about it. In general the faster you spin the more emphasis you put on your cardio vs leg strength. Which is probably why you can see a few of these pro bike riders with twiggy legs (and arms/chest for that matter) as they're just cardio/endurance monsters.