Speaking from personal experience, it is do-able. If the owner is willing to put the time into walks and runs, a Sheltie can make a nice apartment pet.
It goes without saying that every dog needs exercise on a daily basis. Not only does exercise improve overall health and vitality, it can be a very bonding experience for both human and canine.
To say the Sheltie needs an inordinate amount of exercise would not necessarily be true, but a Sheltie does need a good, daily dose of some form of exercise. A long, brisk walk, morning and evening, is often enough. A good game of fetch in the back-yard is also a great form of exercise.
There are classes that teach ‘working out’ with one’s dog. Many performance sports can also be wonderful forms of exercise.
It doesn’t matter what‘s chosen, it only matters that something is chosen. A Sheltie without a physical outlet can, and often will, engage in activities that are undesirable (barking, chewing, etc). Allowing that energy to find a healthy outlet is very important for a happy puppy/dog, and daily exercise is quite helpful to that end.
We have a lovely park in our area where we can run our dogs hard. We also have a back-yard of sufficient size to play fetch, Frisbee, and other chase games. These games, played on a daily basis, allow the excess energy to be released, and make for quiet, happy house dogs.