My rescue mare who was 19 when I started her under saddle was underweight and needed muscle bad. And a worm belly. I was working her about 15-30 min per day 5 days a week for about a month before I took her on her first trail ride. We accidentally went 12 miles because we got lost, she was a trooper about it but boy was she pooped after that.
It will take lots of time especially since she is much older. I would probably just keep it to walking trail rides that don't go to long and just short rides. You can do short rides several times a week to keep her moving and give her a job. And that will be easier on her with her age.