Thank you all for the lovely replies.  We are def lucky to have such a wonderful girl in our lives.

"Tell us more about her daily life. What's her diet like? Her pain management for the arthritis? What keeps her comfortable? Engaged? Happy?"

She is raw fed, prey model diet.  Loves her food!
pain management is one 57g tablet of Previcox a day.  She had been on a different NSAID, but it had upset her stomach.  Since changing to this one stomach issues have gone and she has brightened up so much more, I think it controls her pain much better.  We also give her two Yumove supplement tablets a day.  She was on one a day previously, but we upped it this last year.
She has memory foam beds in each room, but loves nothing better than a blanket to wrap herself up in.... still leaps up onto the bed/couch at every opportunity, cheeky mare. 
When we walk it's a dander along, letting her stop and sniff things for as long as she wants - taking things at her pace.  At home she loves a kong stuffed with mince - we tend to serve them frozen to help them last longer as she's an expert now at flinging them bouncing across the room to knock all the meat out, lol.  She loves squeaky toys.  She used to dyestuff them all the time, but now a days she seems to love the legs off a toy that has squeakers in the feet.  Willow just sits and squeaks away at them and then as a good roll on them.  Playing fetch is just a gentle toss of a toy a few feet away for her to bring back to us; it was always a favourite game of hers, but slower and gentler these days of course.
And of course lots of snuggles, pets and cuddles!