I just finished my freshman year of kinesiology. I like it, most of the stuff they teach is stuff I want to learn.
If you are already lifting weights and into nutrition etc it makes it alot more interesting. You can apply so much of it to what you are doing.
There are the easy classes and tough classes, of course if you enjoy it you don't really think of it in terms of hard and easy as much. You'll probably have to take the anatomy and physiology courses which can be alot of memorizing (bones, agh). I don't know how it gauges in difficultly compared to other majors. I hear it is one of the more difficult ones, but I really don't know. I'd assume the sciences would be more difficult in general.
Like I said before, you are already a step ahead since you are engaging regulary in "fitness". There are bound to be a few classes that are cake because you already understand it all. If you know anyone in the major ask around and find out what profs are good because the teacher really makes all the difference.