Also, I think cycling is physically different enough from most other sports that is seems to bring out some *initial* aches and pains when you first start.
Cycling changed my life, but when I first started - I had aches and pains all over my body. Granted, a lot that had to do with flying over the bars of my MTB all the time (I started racing my first year!), but I had shoulder pains, neck pains, back pains, etc. - but no leg or butt muscle pains!
I find road cycling to cause more pains than off-road riding (now that I spend less time crashing), just because 100psi tires and cracks in concrete get transferred right up through a rigid frame more than roots/rocks on a dirt path with my Rock Shox soaking it all up.
AndrewP is right - a few upper body excercises helps big time. Although cycling doesn't really work out the upper body a lot - it still requires a certain amount of upper body fitness to keep from hurting.