Well it has been several years since I have owned a horse and I am a very casual rider nothing fancy, My guys were always healthy fit and ready to go. So I feel I am a bit green and never had to deal with weight issues (well with horses ......myself always an issue

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I have aquired a wonderful Qh 12 y/o mare. She is perfect she is broke to ride very easy keeper, very forgiving and just super nice. 15 to 15.1 hands
One problem 300 plus lbs over weight easy. She has been well taken care of but exercise is what left her life and started her climb. She is not on grain/feed or hay she is pasture. And has not been for a while or very limited!.
So in reading on diet to decrease or move to a lower calorie hay and feed i understand that. But she has not been on those for almost a year and is still very fat.
Now I have a bare paddock I can keep her off grass, but I am pretty sure it is gonna be exercise that is the number one way to remove her weight. I dont want to stress her or hurt her. Would short calm walks/rides through the fields be ok? Should I just lead her or ride her. Cant run her big girls heart would not make it or mine!
When do I know to kick it up a notch? as with my work out I know because it is my body.
Also with her diet already so limited and her still being so large still how much more can you cut out? and what suppliments do you recommend in order to make up for the food she will or is lacking?