How might this be achieved? A key factor is to monitor the brain to make sure that the user is fully asleep before the exercise programme begins, and also to stop or lessen the exercise programme if the user appears to be waking up.
The exercise itself could be achieved through electrical stimulation of muscles, or through moving limbs via mechanical means (potentially similar to the Puppet Suit concept I wrote about, or some kind of exoskeleton).
It may be possible to develop drugs that deepen sleep to avoid the exercise waking the user. Or to lessen the users sensing of their body.