There are many reasons for a dog to tire quickly but usually it is associated with pain my guess is the dog may be suffering from arthritis. Is the dog old? Has the dog been OFA' d? Yes it can be diet related as well but would look into it if you can find out why you may have avenue' s to increase time afield and also increase her life expectancy especially if it is something serious. Of course the dogs desire level could be real low there are many reasons but health reasons would be my first concern, the dogs health.