You could tie her while the other mare is eating. But I think you'll see the most improvement by setting up a gradual workout schedule for her. I wouldn't lunge her for longer than 30 minutes since small circles can be hard on a horse.
EDIT:: (I didn't see that you had posted again, sorry. ;))
Be sure not to run her too hard, especially if she isn't used to hard work. With the fatty that I started conditioning in spring 2009, I started off with only walk and trot for half an hour and gradually built up our rides.
Just be sure to cool her off before feeding. ;) What that means for me is that the horse's skin is warm or cool to the touch (I usally feel the chest area) and that they are breathing normally, as opposed to flaring their nostrils for breath.
Last edited by IslandWave; 01-21-2010 at 02:13 AM.