I've had to do this a couple of times, over the years. Hill work was the easiest way for me to get some condition on. Don't know where you are, but if you can find some gentle slopes (no more than, say, 1/4 mile, at first) with a mild grade, try slowly walking up and down the slope, a few times a week. Do that for a couple of weeks. Assuming that s/he's bearing up well, try to find a longer slope or a slightly stiffer slope (sharper grade) and do the same thing. Over time, you can increase the frequency of hill work (more times a week, say) or go up/down the same slope multiple times, or ramp up to trotting the hills, etc.
Also, swimming is a nonstressful way to put condition on a dog (well, a horse too, but that's another story). It might be worth it to call around and see if nearby vets know of a facility that will let you therapeutically 'swim' your dog. (I once sweet-talked an equine hydrotherapy facility into letting me use their space for an IW recovering from back surgery, which they graciously did. Worked wonders for her and she loved it.). By the way, I'm assuming that this is just a matter of building muscle mass, and that you and your vet feel that the dog is okay for more demanding exercise than gentle walks.