Hi Denise! Welcome! And don't worry - MrsO and I won't be disappearing any time soon! Well, I hope not anyway! MrsO is one of the people who has got through to the other side of PMR and me - well, not there yet but getting there slowly!
As MrsO has said, methotrexate is a standard RA drug and prevents the joint damage you get with RA but which we don't have luckily. It can also have an effect on the way the body metabolises pred so that in some diseases a smaller dose has the same effect as a larger one if you take both together. There have been some studies done in the past that seemed to suggest this was the case with PMR - the latest thinking in the last couple of years is that it DOESN'T help enough in PMR to make the side-effects worth it. Some rheumys are still keen n using it if they think you are not fitting their ideas of how quickly you can stop pred.
Let's deal with the next lot of questions:
Exercise exacerbates PMR in that if you try to do too much the muscles take much longer to recover afterwards so you feel sore and stiff as if you had gone for a long run or skiing or something without training. It isn't known why, I suspect it is because the tiny blood vessels that supply the muscles are not allowing blood through because of the swelling and inflammation so the metabolic products of exercise stay in the muscles for far longer (basically the lactic acid in exercise story). What you should be able to do is find the amount you CAN do without feeling too bad by starting with a small amount of walking and then building it up very slowly, never pushing it too far. That'll do there for now - except to say, don't EVER let anyone try to persuade you to go to the gym or something on the grounds it will "cure" the PMR. It won't, you'll just feel rubbish afterwards. Everyone is different, some can exercise, some can't. Know yourself!
Low carb - what that WILL help with is avoiding weight gain because of pred. No diets have been found to help 100% with PMR - including as many anti-inflammatory things in your diet as you can probably will help ease symptoms, MrsO is the expert there and she found using certain things did make a noticeable difference as to how she felt though it didn't "cure" the PMR. Gluten-free isn't a definite answer either - I was eating gluten-free when mine started - and using gluten-free foods adds calories and carbs (and cost) in large quantities! I do find that when I eat grains I feel different - but I almost never eat wheat anyway as I'm allergic to something in it.
Fatigue is a symptom of almost all autoimmune illnesses, PMR is the result of an autoimmune disorder. Some people find pred makes them tired, others find it make them hyper. It all depends, everyone is different (that's something you'll hear a lot). Unfortunately, a lot of doctors struggle with that concept, they want us to fit a pattern.
You say you know you can "trust the picture painted by people who have been through the experience" - you will still find some strange and unfounded claims because people will latch on to forums like this if they have an agenda of their own. Others have a bad experience and think it is the same for everyone and try to push their story to the fore. You still need your eyes open and your brain in gear! This forum is usually good because a close eye is kept on it. There is another forum run by the PMR and GCA UK Northeast Support Group which is also very reliable and has a lot of info on it - we are so closely allied to the UK charity that we can't afford to have wibble and it is moderated VERY closely. The national group also hosts a forum - I have no idea how good or bad it is, just it is there. You'll find a few people who are closely involved with all of them.
Ask away if you have any other questions and we'll do our best to answer them. You say you haven't been diagnosed yet - has your doctor also considered vit D deficiency which can also lead to very similar symptoms in some people. It should be checked when making the differential diagnosis - there are several things that appear very similar to PMR which need to be ruled out. The main difference, however, is that a 15mg/day dose of pred will give a lot of relief from the symptoms (not 100% at first but a lot) within 48 hours. Stop the pred after a week and the symptoms come back. The group in Bristol use that as a "test" for simple PMR (i.e. not complicated with something else).
Eileen