Yeah, you're just not used to it; many riders hover in the 75-90 rpm range normally. 45-60 is what some people call "mashing" the pedals, and they feel endurance suffers with the practice. It MAY WELL have an ill effect on your knees over the long term.... I don't know, I'm in the mid-80's most of the time.

Don't take it the wrong way, but you sound like my sister with that cadence; she mashes, but can top 28mph on fat tires and crap suspension. She can also fix her own cars, move furniture with any man, and is the best mother to her 6 kids I've ever encountered.