Do you have a dry lot area you can put her in? If not, if you can do a makeshift small fenced area, and mow the grass almost down to the dirt, it will become a "dry lot" in no time at all. I always found putting them on dry lot allowed me to control their intake. Then you can move the hay around so she has to walk to eat, and she'll get some exercise while she's nibbling. Then, I would start taking her for walks. Start out on short walks if she's out of condition -- we don't want to disturb that little one -- but she needs to be in good form for the hard work of delivery.
As we near winter, the extra fat will be a benefit in helping her keep warm, and I would imagine with some dry lot, controlled intake, walks and the cold of winter, she'll be in good form for spring and delivery!
Keep us posted!