Weight gain with pred is common but not inevitable. It changes the way our bodies process carbohydrate so cutting them right from the start does help as well as reducing the risk of developing steroid-induced diabetes. Some weight gain may be from fluid retention - cutting salt is suggested to help with that.
This may help you understand how to taper in PMR:
http://www.rcpe.ac.uk/sites/default/files/quick.pdf
although for some people the steps are a bit big and they have problems as a result.
This
has helped a lot of people cope with reducing by slowing down the rate of change for any given step down.
Cushing’s syndrome is the name given to the effects of excess corticosteroids in the body - it should improve once you get down to about 7mg which is a similar amount to what the body produces naturally in the form of cortisol and which it needs to function. Keeping your weight down does make a big difference to appearance - I lost 35lbs of PMR and pred related weight gain when I cut carbs and even the doctors say there is no sign I am on pred, and have been for over 10 years.