I would also think she wouldn't be able to run 2 hours a day if she was less than half of her ideal body weight. :/ Hopefully that was an exaggeration.

What kibble are you feeding her? If it is based on fillers, it'll go right through her, and for some dogs that means they'll be unable to keep weight on. Go for a high-meat kibble such as Canidae or Innova Evo. Add supplemental meats, and give her a satin ball or two every day (do a search for "satin balls" to find out the recipe). You want her to gradually put the weight on and keep it on. If you're able to, raw feeding helps many dogs keep a healthy weight on.