I am just coming out of this condition. It's odd, and the medics don't know why, but it often lasts only 2 years or so with steroid treatment.
Once you're on the steroids there will be an almost magical transformation - all your aches and pains will disappear very quickly. I was pain free within 48 hours. But you have to remember that steroids subdue all pain, not just that caused by the PMR, so when you eventually come off the steroids, or when you have reduced the dose dramatically. the normal arthritic pains will return.
You will also have to take calcium and vitamin D tablets daily because steroids increase your risk of osteoporosis. Ask for a bone density test to see the condition of your bones now. I had one at the beginning and another a few months ago and my BD has been fine throughout.
There are possible side effects from the steroids just as there are with all drugs. Make sure you read the info leaflet thoroughly. The worst side effects I've had are weight gain (but it doesn't take much with me - I do like my food!) and an increase in sweating, mainly through my head - while on higher dosages (8-15mg), I would suddenly and for no apparent reason feel sweat trickling down my face and neck. I grew my fringe out because it was frequently getting drenched
. Since I got the dosage down to below 7mg this side effect has gone.
Please don't worry, you will probably be absolutely fine and it will be so wonderful to be pain free!