How long till I exercise

How long until I can start to exercise Dixie again. I had cut back towards the end of her pregnancy, and cut it out all together minus romping after a baseball in the back yard after the pups were born. But I would like to get her going again, as it cannot be healthy to hang out and feed pups 24 hours a day without walking/running and swimming.Those were her old pastime favorites. How much exercise would you suggest? I know the usual routine of 2 or 3 miles a day is prolly too much to start with again,and I dont want her using too many nutrients and such in exercising, because I know she needs them for the babies sake. She is having a hard time putting weight back on. I have had to supplement her with extras and let her eat as much as she can at feedings, and up'd her feedings to more often per day as well.Im kinda at a loss because I am doing what the vet recommends, but she is not responding as much as Id like.

Thanks for your advice...