Horseback Riding As Exercise?

I have been thinking lately about different ways I can exercise that would be fun.  A blogger friend of mine has been writing a lot lately about her experiences horse back riding, and it sounds like so much fun.  I don't know much about horse supplies or how much work goes into taking care of horses, but I bet it is more fun than going to the gym and walking on a treadmill.  I guess I need to do a little research, and see if there is somewhere around here I could try it out before I commit to anything.  Who knows, I may get thrown off and break some bones my first time out, or I may find out that I am really good at it.  I guess we'll just have to wait and see.