Why not try contacting a couple of horse riding stables now? You might think your overweight but explain your situation to them and i'm sure they'll be supportive.

Every stables i've been to have been really keen to get people into horses as they know how much fun it can be and would make sure that you were comfortable with the horse your were given and the lesson you were put into. You'll find you'll meet a lot of people like you and as it was mentioned before once you start riding you'll lose weight in no time! its a great form of exercise!

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Hunky Horse
www.hunkyhorse.com