Slowy increasing the intensity, according to the horses comfortable athletic level is really the only way. There is no quick fix to lost muscle tone that I know off, other than slowly building them back up.
So, since it's been a couple of months, start off with perhaps a long walk/trot ride. Then do that a few rides, slowly making the trot amount longer each time. Then, start introducing in the canter, and start building from there, short sessions at a time.
"You're just as sane as I am."~Luna Lovegood.