Yes, people do get over PMR and GCA and back to "normal", although since you are older and likely to be less fit by the time it happens it may not be like you felt immediately pre PMR. Remember you are "meeting" a skewed population here - the people who have problems and/or have only recently been diagnosed. Most people who don't have problems aren't here - they don't need the forum. MrsO is over her GCA and I am pretty much over my PMR - we're still here because it is our "work" in the voluntary sector, so to speak. She runs a support group in the south of England and I do this and work with a research group in an expert patient capacity.
About a quarter are over it and off pred in about 2 years, sometimes less, half take up to 4 to 6 years and the rest are longer. I'm in the final quarter after 10 years since the first symptoms appeared. I'm on 4mg pred but feel pretty much fine. I've given up a few things but I probably would have given some of them up by now anyway. In the meantime I've had major flares and been on different sorts of pred, some OK, one definitely not OK and the current one is really good for me. I put on weight - I have lost it all and a bit more and am just about back to my pre-PMR weight (I put on a lot with PMR itself because of finding it difficult to exercise because of no treatment for 5 years because it wasn't recognised, I was only 51 when it first appeared).
Keeping carbs to a minimum will help avoid the weight gain and the diabetes risk. Once you have the inflammation under control and learn to pace yourself you will feel much better. You will learn to manage your life to accommodate PMR - and believe me, it is far easier on pred than without it! Given the choice of being chubby or being in constant pain, for me there was no choice. I found using other therpies made life more comfortable - in particular I very occasionally used osteopathy (chiropractic but not the woo variety with supplements and weekly visits!) and, above all, Bowen therapy. I did Pilates for that first 5 years - that and Bowen kept me upright with no pred until it went absolutely haywire and I was finally diagnosed and given pred - 15mg achieved a miracle in 6 hours. Some of the pain you have can be improved by targeted therapy - cortisone shots and Bowen in particular - and that probably allows a lower oral dose.
An antiinflammatory diet - adding things like oily fish, turmeric, garlic rather than omitting things seems to help some people. I've tried gluten-free, alcohol-free, nightshade veggie-free - none made the slightest difference. Very low carb HAS allowed me to lose 40 lbs in weight even though I was still on pred. It also helps with fluid retention which pred does tend to cause.
But if you are able to walk over 6 miles - you aren't doing badly. I had a year when I couldn't walk more than a few hundred yards and that was on crutches and even before that a couple of miles was about the limit. No idea how far I can walk now - for a few hours at my own pace I suppose.
I still believe the most important aspect is acceptance - that isn't giving in, it is seeing you are where you are and not fighting it. That uses up so much energy that you can use far more productively elsewhere. Yes, we all feel sad at what isn't possible at the moment - but the people who get through this best learn to live with it and find new things to do. One lady took up making cards in the long night hours when she couldn't sleep - she has raised a lot of money for her animal charity and for PMRGCAUK NE as a result. She had been doing youth work, walking, abseiling and so on until PMR hit. Others of us work with the charity in various ways and most of us had been active and fit people beforehand.
I'll leave it to others to tell you their experiences of mtx - there is no evidence it helps reduce pred reliably or significantly in PMR. PMR can often be confused with late onset RA, however careful the diagnostic procedure, because there are NO 100% sure differences - maybe the people for whom mtx works well have that or both. As a "steroid sparer" it is thought to change the way your body metabolises the pred so that a smaller dose achieves the same effect. Some people have used it and it appeared to work at first as they reduced their dose only for them to have a big flare later.
You sound to be doing not too badly at the 6mg dose but your doctor appears to want to get the inflammation under control - if you use a higher dose to do that and then reduce carefully (ie in small steps) back to 6mg you may get away with it - 10mg is commonly plenty for many of us as the maintenance dose and by reducing very slowly most people who have tried the "dead slow and nearly stop" reduction plan have got well below it. The biggest mistake you can make is to try to rush reducing and going in big steps - that way lies trouble and you get into a yoyo pattern with the dose.
Loads of links to info here (sorry if I've already given it but better once too much than once too few):
https://patient.info/forums/discuss/pmr-gca-website-addresses-and-resources-35316
and the slow slow reduction is there too.