Arthritis in a 5 year old is a pretty good indication that she'll only ever be suitable for very light riding.
A career as a trail horse may be her best option.
She'll also be better in regular, steady, light work - like 30 - 45 minutes daily, without intervals where she isn't ridden. This type of work actually keeps them limber and moving and slows the progress of the arthritis.
Ring work, or any work that involves circles or turns, is probably not going to be best for her.
There are a lot of maintence options available for arthritic horses - oral supplements, oral meds and injectables such as Legend, Adequan and Cosequin.
24 hour turn out, *light* steady work, maintenence and cross your fingers.