"John W." wrote ...
> I've ridden horses a lot in my life, and you do get sore muscles, but I
> wouldn't call it exercise.

Well, theoretically, if you lay down on a bed and someone hit you with a 
padded club you might get sore muscles - but any other mechanism 
for producing muscular soreness pretty much has to be 'exercise' doesn't
it?

Oh yes ... thanks be to Google 
http://www.netfit.co.uk/fatcal.htm

At a rough approximation horse riding is about as much 'exercise' as 
swiming (slow) or skating (inline).  Obviously it's not running at 10mph
but who wants to do that?